What happens when a tripped (prone) target is grappled or a grappled (grabbed or restrained) target is tripped?
Do you just stack the conditions? If a restrained target is tripped, does it break the grapple? Does the restraining character also become prone?
If prone then grappled, you're both. You get the drawbacks of both, but you're only flat-footed, not double-flat-footed if that's what you mean by stack. In PF2, penalties of the same type don't stack, so the -2 circumstance penalty twice wouldn't add to -4 anyway.
Same w/ the inverse order of course.
Tripping doesn't remove other conditions, nor have an effect on the restraining creature.
The exception would be if you move the target so far away that the grappler couldn't reach their target any more (though I believe you have to overcome the grappler's DC to do this.)