I guess it depends on what you would consider cheating.
Can they have sex with someone else while you’re in a separation? Certainly, there’s a good chance you’ll even feel it. They can get married to someone else.
They might do it because they’re entirely closed to the spiritual part of the journey, they might do it because they can feel it, but they’re dealing with soulshock, so they’re trying to root themselves in something purely physical in the 3D to try and cope.
None of that erases the twin flame journey.
If you mean can your twin cheat on you when you’re in union?
I don’t believe so. Potentially, if you’ve jumped the gun a bit and you’re in the earlier stages before truly going through the work to be ready, then you could be dating and they sleep with someone else (same reason as the twin flames being toxic thread), but I don’t believe anyone truly in union would have to worry about cheating.
Danielle has a guide and video here, too.