How much of a Devil Fruit must be consumed to gain the power? To get the devil fruit power is it necessary to eat the whole devil fruit? What if one person eats half and some other person eats the rest? Will both of them get the devil fruit power? 

I am currently on the Dressrosa Arc. So, please try to give a spoiler-free answer.
It is not necessary to eat the entire fruit. A single bite is enough for the consumer to obtain the power. Oda addressed this in Volume 77's SBS corner. 

Source: https://onepiece.fandom.com/wiki/SBS_Volume_77

During the CP9 arc, Jabra also states that "one bite will leave you with a lifetime of problems", when warning Kalifa and Kaku about Devil Fruits.

If two people split the fruit and share it, the person who takes the first bite will obtain the power. This was illustrated with Ace's story on how he got the Flame-Flame Fruit.