All chinchillas are different ... each has their own special personalities. As with humans, some personalities conflict ... between two chinchillas and sometimes even between a chinchilla and a human. Have you ever liked someone, but they did not seem to like you. Or someone wanted to be friends, but you just didn't like them? This looks to be the case between your two chins.
Looks like you may have two dominate females. If you have ever heard the ole saying, "Two women can't live in the same house." This may be the case with these two particular two female chins.
The "mounting" is very often NOT a sexual thing ... it is a show of dominance ... trying to decide who will be the alpha or boss chin. If both have a strong personality, you may not be able to get them to stay together peacefully. You may have to invest in a second cage for the safety of both chins and your sanity.
The time-out carrier is a good way to show them they must behave during playtime, but once you leave ... if they are in the same cage ... you may come back to one or two injured chins or even a dead one. I don't mean to scare you, but, I have seen it happen.
Keep us posted!
Jo Ann
There are no dumb questions ...
Only regrets they were not ask ...