Here is some info about cohabitation:

Cohabitation:

What's better than one turtle? 🐢
That's right - two turtles 🐢🐢 😁😎

Just not in the same tank.

Cohabitating aquatic turtles is a bad idea. They are not social animals, don't need friends, and don't get lonely. Living with another turtle generally causes them stress, as there's now competition for food, basking space/heat, and more. It also creates a higher bio-load on the tank & filtration, which can lead to substandard water quality & cause stress/health issues. Bullying and passive aggressive behavior are common, and often go unrecognized or mistaken for "cute" and "friendly/cuddle-y" behavior. Make no mistake, no matter how long they have lived together "peacefully", they have merely tolerated each other. This is a ticking time-bomb, and can change in an instant, often without warning (example). Eventually... violent aggression/attacks are not only likely, but a near-certainty.

There is no safe combination. Male + Male is highly dangerous. Male + Female = potential breeding aggression (Example, Example, example), but females may also attack males (example). Female + Female is still risky (example). Age doesn't matter (Example, Example). Different breeds does not necessarily negate this problem (example, example). Even the biggest of tanks is not a guarantee either (example).

There is no evidence, to date, of benefit to cohabitation (for the turtles), and ample evidence (including anecdotal) of risk and harm. As such, cohabitation is not worth the risk, and should be avoided (as is reflected by most reputable husbandry guides & experts).

Cohabitated turtles should be separated to their own enclosures immediately. Waiting to separate until seeing signs of aggression is a bad idea; we do not watch our turtles 24/7 (fine when watching, attack after leaving). It is easy to miss something, and things can turn bad at any moment.