You shouldn't introduce them at all if one is male and the other is female, unless one has been fixed months ago, at the very least. Even if you are watching over them, it will only take them moments to mate. Chins shouldn't be bred unless you know their histories and lineage and that these two are ideal for mating. They need to remain in separate cages and have separate play times. Also, even if they were of the same sex, they may never get along. Chins have died from fight related injuries. Last thought, I can't tell if you have had them longer than a few days, but if you have only had them for such a short period of time, their cages shouldn't have been put together as you want to do a 30 day quarantine first to make sure one chin doesn't pass an illness on to the other.
Not meant to judge, only to inform. To answer your question, just give them separate play times, always, no exceptions. If you are worried about them needing a friend, get them each a same sex partner, or get them a cuddle buddy - fleece toy - to give them company.