Obligation of treating co-wives fairly and some of the rules on travel for men with more than one wife
Question

I would like to know if it is allowed for a man, when he is married to two wives to take the second wife every time if he will travel, even if the first wife cannot go because of her children. What can she do if she feels that he do not want to divide his time equally between them. Please advice a good website with answers on polygamy.
Praise be to Allah.Firstly: 
Allah has enjoined justice and fairness in all things. He says (interpretation of the meaning): 
“Allah commands justice [and] the doing of good” [al-Nahl 16:90]
Ibn Jareer al-Tabari (may Allah have mercy on him) said: 
In this Book that He revealed to you, O Muhammad, Allah enjoins justice, which is fairness."(Tafseer al-Tabari  17/279). 
Allah has forbidden injustice and wrongdoing (zulm) to His slaves, and He warns those who are unjust or do wrong of punishment in this world and in the Hereafter. 
It was narrated from Abu Dharr (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said, narrating from Allah, may He be glorified and exalted: “ ‘O My slaves, I have forbidden zulm to Myself and I have made it haraam among you, so do not wrong one another.” Narrated by Muslim (2577). 
Allah has enjoined justice and fairness between co-wives, and there is a warning against wronging one of them at the expense of another. Allah says (interpretation of the meaning): 
“And if you fear that you shall not be able to deal justly with the orphan girls then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess. That is nearer to prevent you from doing injustice” [al-Nisa’ 4:3]
Shaykh ‘Abd al-Rahmaan al-Sa’di (may Allah have mercy on him) said: 
i.e., the one who wants to take two or three or four wives may do so, but no more than that, because the context of the verse reminds people of the blessings of Allah, so it is not permissible to add anything to the number that Allah has defined, according to scholarly consensus. That is because a man’s desire may not be fulfilled by one wife, so it is permitted for him to take more, up to four, because four is sufficient for anyone, except in rare cases. However, that is only permitted to him if he is confident that he will not be unfair and unjust, and that he will be able to give them their rights. 
If he is afraid that any of this applies to him, then he should limit himself to one, or to slaves that his right hand possesses, because he is not obliged to divide his time equally when it comes to slave women. 
“That”, i.e., limiting yourself to one or to what your right hand possesses (slave women) 
“is nearer to prevent you from doing injustice” i.e., being unfair or unjust. 
Limiting it to one applies in the case where a person fears he may be unjust or unfair, and may fail to do what is required, so he should err on the side of caution and not put himself in that position."(Tafseer al-Sa’di  p. 163). 
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Whoever has two wives and favours one of them over the other, will come on the Day of Resurrection with one of his sides leaning.” Narrated by al-Tirmidhi (1141), Abu Dawood (2133), al-Nasaa’i (3942) and Ibn Majaah (1969). Classed as saheeh by al-Albaani in Saheeh al-Targheeb wa’l-Tarheeb (no. 1949). 
Shaykh al-Mubaarakfoori (may Allah have mercy on him) said: 
Al-Teebi said in his commentary on the words “with one of his sides leaning”, i.e., tilting. And it was said that this will be in such a way that all the people on the Day of Resurrection will see him, so this will increase his punishment."(Tuhfat al-Ahwadhi  4/248). 
If a wife sees that her husband is favouring her co-wife at her expense, or is being unjust to her with regard to her rights, she should hasten to advise her husband in the way that is best, and remind him of what Allah has enjoined of justice, and what Allah has forbidden of injustice. She should also hasten to advise her co-wife not to accept this injustice, and not to take anything that is not rightfully hers. Perhaps Allah will guide him to be just and to give each one her due rights. 
Secondly: 
One aspect of justice between co-wives is for the husband to draw lots if he wants to travel with one wife and not the other(s). This is what the Prophet (peace and blessings of Allah be upon him) did with his wives.  
It was narrated that ‘Aa’ishah (may Allah be pleased with her) said: When the Messenger of Allah (peace and blessings of Allah be upon him) wanted to go out on a journey, he would cast lots between his wives and the one whose name was drawn, he would take her with him.
Narrated by al-Bukhari (2454) and Muslim (2770). 
Al-Nawawi (may Allah have mercy on him) said: This shows that if a man wants to travel with one of his wives, he should cast lots between them. In our view this casting lots is obligatory."(Sharh Muslim  15/210). 
Ibn Hazm (may Allah have mercy on him) said: 
It is not permissible for him to choose one of his wives to travel with him except by drawing lots."(Al-Muhalla 9/212). 
Something similar was said by al-Shawkaani (may Allah have mercy on him) in al-Sayl al-Jiraar (2/304). 
When he comes back from his trip, he should not count the period of his trip for the wife who travelled with him as the result of drawing lots.  
‘Abd al-Barr (may Allah have mercy on him) said: 
When he comes back from his journey, he should resume the division of his time among them, and he should not count the days of the one with whom he travelled. The hardship that the wife who accompanied him went through as the result of travelling is in return for the time she got to spend with him."(Al-Tamheed  19/266). 
Thirdly: 
If it is assumed that one of his wives was not able to travel with him, it is pointless to include her in drawing lots, when she is unable to travel with him. In that case, lots should be drawn among those whose circumstances allow them to travel, and lots should not be drawn between those who are able and those who are not. But this is based on the assumption that this is the truth and not just an illusion or based on mistreatment of her, such as if she is sick or she has children and cannot leave them without someone to look after them, or she is not allowed to travel, and other such reasons, and it is not because he wants the other wife to travel with him and not the first one. Otherwise he is being unjust. 
In this case he has to try to please both wives, even if it means making it up to the one who is not travelling by spending extra time with her when he comes back from his trip. 
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 
Al-Qurtubi said: That varies according to women’s situations, and the prescription of drawing lots applies only if their situations are the same, lest one of them go out with him without a reason to make him take her and not the other(s)."(Fath al-Bari  9/311). 
Dr. Ahmad al-Rayyaan said: 
If the wives’ situations are the same in all aspects, then drawing lots is a must. But if his wives differ in that, there is nothing wrong with choosing, so long as he pays attention to the principles of not favouring one of them and not aiming to cause harm."(Ta’addud al-Zawjaat  p. 71). 
We do not know of any site that deals specifically with issues of plural marriage. You can look at our site, and at other trustworthy fatwa sites which deal with a lot of questions about plural marriage. 
We have compiled a separate section on our site dealing with issues and rulings on plural marriage, and you can find it at this link.
And Allah knows best.