In most RtwP actions do not really happen simultaneously. They actually follow the initiative rules of PnP and have rounds systems. Certain things like movement is free, deviating from PnP, but all other actions aren't. I'll say though, what you suggest does make me think that it would be interesting to have a rules light, "simulationist" CRPG, one that discard rounds and AoO, uses physics and hitboxes, and tries to make a combat system that tries to simulate "realistic" fantasy combat. I think maybe Total War kind of do this, but it is a RTSwP, and maybe Spellforce offers a gameplay similar to what you suggest (although it looks too light on the RPG side of things). None of them are proper RPGs though, so maybe someone has a better example than those.