Opening 2 tunnels for 2 NICs on the same machine

Hey everyone,

I am working at a company that needs clients to go out to the internet with a 4G connection.

I have a Raspberry pi 4 with Squid proxy running which is listening on 2 ports, for example - 3001 and 4001.

I would like to use a cloud service like AWS to open 2 different tunnels between the cloud and the Raspberry, and then open some ports on the cloud so I can reach the Raspberry from anywhere without forwarding any ports on the rasp itself (which is impossible on a mobile network in my region).

Traffic will take this route:

User on port 3001 -> AWS on port 3001 -> Raspberry on port 3001

User on port 4001 -> AWS on port 4001 -> Raspberry on port 4001

I know how to achieve this easily by using a VPN, but the issue is that I have 2 NICs on this Raspberry (USB modems), and using a VPN only opens one tunnel and is using only one NIC, creating a bottleneck.

how can I open 2 tunnels, each using a different NIC on the Raspberry?

This way "user3001" will have the full bandwidth of 1 modem, and "user4001" will have the full bandwidth of the other modem.

Thanks!