"/revmode soft" indeed only stops the timer from ticking, and that is exactly the intended effect - you can take as long as you want to gather the required health to revive the coop partner, and even leave the level if you can't, which will drop them off at the next level's starting point.

If the game was to not divide health for revival, there wouldn't be any penalty for dying, which would mean that you could stay close and go as far as you want, so long as both players aren't killed at the same exact moment.

If you are not keen on sharing health like that, you can also try "/revmode old", which will revive the dead player whenever opening a health chest.