In this way, people with a lot of stake won't see voting on comments as an opportunity cost.
I don't know this is a problem. If there isn't enough voting on comments, then the value of a vote on a comment will be higher than the value of a vote on a post. This should reach an equilibrium even with shared voting power. I haven't thought a lot about the behavior with combined or separate vote powers. There are probably indeed some important differences, but I don't think separating vote power is clearly necessary.