I just thought that it would be great if we could create a replay in single player, then bring it into a multiplayer lobby and the players could start at a certain frame and continue at that spot. If this has already been suggested, I was too lazy to check. We could also do it with other multiplayer replays.

Supported.

For long I've wanted to finish my multiplayer parkours where one of the parties left.

I think there is a way of continuing n MP replay in SP from any point, although i dont know how to do it.

Watch moop's second tk mind games video, he does it there, (still dont know how)

Here's your solution:
http://forum.toribash.com/showthread.php?t=452268

It will work for your said intentions as well ;)

But then how will both player get control in it at the same time?