I recently adopted two male brother chins from a pet store. They are both six months old. I originally purchased the first one and then after two days got the second one as I thought it would be better for the both of them. The first one is Niko aa timid but super friendly chin and the second is loki a more adventerous and mischivious chin. Loki has started to blossom into a fun energetic curious chin while Niko seems to want to hide away in the cage and not come out to run and play when it is time. I was wondering if its possible loki is being dominant, or maybe niko suddenly doesnt trust me? Either way I just want niko to come out of his shell like his brother because he was on his way to being even more people friendly but now he hides away in his spot in the cage and would rather stay then come out. What do I do?!?!

Congrats on your little friends
I don't know about chins but it is possible they were only cage mates, not biological brothers. Pet stores run by buying pets en masse from breeding mills. Goodness knows what the actual genetics are of your little guys.
As long as the cage is huge enough for both to have their own space if they want it and there are no fighting or constant bickering then I would say it's fine. Keep an eye on them though. The shy one might come around, as I said, he comes from a mill most likely and therefore has had little good human contact. So just give him time.

chins take time to learn to trust you, remember they are prey in the wild.
try just putting your hand in the cage, and wait for hiim to come to you, it will take awhile, you could also put a treat on your hand, let him realize you aren't going to hurt him.