Police A and Police B who police each other besides the ordinary citizens. Add incentives so that they look extra closely at the other side.
The alternative is two sides fighting a war of escalation which would likely encourage scary corrupt behavior. "Those guys busted Johnny for some minor offense, so lets go get some dirt on one of their guys, or plant it. Can't let them think they can just mess with us"
This is going by the current us vs them mentality of the police forces.
If that is so, then at least if both "us" and "them" are heavily armed cops with warrior mentalities, they'd focus less of their aggression on the general public.
/s
Don't physically try to stop the police. That's going to cause trouble. But if they're doing something out of line, record it if you can. If they tell you to stop, you can claim that you have the legal right to do so. If they continue to tell you to stop, stop. But still watch if you can.
Note well: IANAL. This is not legal advice. In particular, if anyone with real knowledge wants to correct the "If they tell you to stop, you can claim that you have the legal right to do so" part, feel free.
This is the metapolice we're talking about, not the police. The metapolice have no jurisdiction over ordinary citizens. Now, if police were to interfere in lawful metapolice activity, that would be one hell of a crime.