@madbrain Well, I guess it doesn't really matter what may or may not be possible if no one is doing it. And like I said, I don't think you will find any VPN provider that does multistream VPN.
So your are stuck with single stream and no possibility to increase throughput through your load balancing setup.
What you can do however, is to create a LAN Pass rule like this:
b6f63c5c-c266-45cf-a680-12fdd43bdc65-image.png
Select Protocol TCP or TCP/UDP, which lets you specify the port further down.
YourLANIP in this case is the IP of your PC that is running the Wireguard client.
And VPNPort is the port being used for that clients' VPN traffic (or ports if there are different ports for different endpoints).
And under Advanced Options, near the bottom, you select the Gateway you want that traffic to exit via.
That's it...
What you don't have with this is the failover capability for the VPN client. Since your policy rule is forcing all the traffic via one of the interfaces, and if that interface is down, you are stuck. I don't know if there is a way to get around this, like prioritizing the faster interface in the loadbalancer.