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 ;)

Respar can help u, but i used it already and, in some matches i tried to finish an uncomplete replay with a friend it just doesnt worked 100%.

I feel like this would be useless for single player replays. What's the purpose of continuing a sp replay in multiplayer, because it's a single player replay.
This should probably only be used for multiplayer replays.

better collabs
for fun

just put it in mediafire or something

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

both people getting control? why