I thought about something like that to make video calls more stable. For example I'll get completely different routes (sometimes noticeably lower/more predictable latency) when I use a VPN to connect to some peer in the US (from Europe.) Would be cool to combine different routes.
One difficulty with using multiple routes is you'll probably need to spend a lot of bytes on active probing, because the quality of a connection may change during the call and when the active connection loses quality it's not apparent what the other connections will do, so you need recent probing from them as well to make a smart change.
But in theory, you should have several routing options in a well supported calling service. I'll illustrate one direction, but the same options apply in the other direction, and there's no need for both peers to use the same connection to send (although WebRTC will)
Peer A -> Peer B
Peer A -> Relay near A -> Peer B
Peer A -> Relay near B -> Peer B
Peer A -> Relay near A -> Relay near B -> Peer B
If at least two of the four hosts mentioned have IPv4 and IPv6, you can also add those permutations. It's pretty typical to have different routing for v4 and v6.
