How does the "Chakra induction paper" react when someone has multiple chakra natures A Chakra induction paper can be used to detect an individuals Chakra affinity. In cases where a person is born with affinity to multiple Chakra Natures, do all those characteristics happen simultaneously to the "induction paper"? Or does the paper react only to an individuals strongest Chakra Nature. If so then how do Jonin level Shinobi check for their secondary Chakra Natures or decide what other elements would be a good fit for them.
In cases where a person is born with affinity to multiple Chakra Natures, do all those characteristics happen simultaneously to the "induction paper"? It is not known or rather, it was never shown in the manga as far as I know. In Chapter 315, it was not stated whether the litmus paper test is applicable to people using two nature types. However, it is implied that any person only has one nature affinity, regardless if they have Kekkei Genkai or not, or if they can use multiple natures. As far as I know, there were no people mentioned that have multiple chakra nature affinities, only those who can wield multiple ones. The existence of the litmus paper test in itself is proof that people only have one nature affinity. They would've thought of a way for another test if ever there was a person with two nature affinities. So yes, the paper reacts only to an individual's strongest chakra nature or the nature which he/she has a strong affinity towards.

How do Jonin level Shinobi check for their secondary Chakra Natures or decide what other elements would be a good fit for them? Again, this was never stated nor elaborated in Chapter 316 so we can only guess. However, if I were to guess, it was never the case that they have to find out their second strongest affinity to another nature but rather, they would probably choose a second nature to master that would give them an advantage, especially on situational cases. Also, they would probably master a secondary nature that is strong against a nature they are weak on. For example, if one has an affinity towards or if they have their strongest jutsu in Fire Release, they would probably master Earth Release to counteract against Water Release. Or it would be possible that they will just master any element they can. The only thing that can be said is that we can never truly say that a certain chakra nature is a good or bad fit to another certain chakra nature, or that you shouldn't master a certain nature if you possess or have an affinity towards another certain nature since it depends on many things.

Apparently, you can master all releases through training. Examples of shinobi who did so are Hashirama Senju, Tobirama Senju, Hiruzen Sarutobi, Orochimaru and Mū. Other possible ways to acquire master all chakra nature include: 


Possession of the Rinnegan to shorten the learning process.
Acquiring the Six Paths Senjutsu and Truth-Seeking Balls.
Possession of the Bashōsen allows any shinobi the usage of all five
natures.
Using Earth Grudge Fear to gather hearts with other chakra natures (Kakuzu)