multiple VPN connections from the same device

I have set a VPN server (ubuntu) with a docker from the kyle/manna project. I want to establish two different VPN tunnels (VPN tunnel 1 and VPN tunnel 2) from another device (ubuntu) which owns two wireless connection (two LTE modems). Then the device which will have the VPN client will have two different interfaces called wwan0 and wwan1. How can I establish two different VPN tunnels? How can I send traffic via one VPN tunnel or the other? In other words, I want both tunnels to be connected to the same server via different interfaces at the same time. The reason is that each tunnel will send the traffic through different cellular networks. It could happen that pne cellular network does not work at a moment and the other one does. Then the traffic could be sent via the one working