I think this is a bit of a problem. Not to be negative but if Grace is supposed to be the future of Resident Evil, she has HUGE uphill battle to win us over if you're going to have the fan favorite share equal time with her. I don't see how she won't be completely overshadowed by him.
Also if Grace is going to be slow horror and Leon fast action, I can see a situation where this could end up being an uneven game, with some people disliking half the game because it doesn't cater to their style. Maybe they'll pull it off, but we'll see.