The easiest method is to connect the two routers using a dedicated, routed port on a small transfer subnet (/30 or /31) and add a static route to the remote network on each side. If network addresses overlap you need to renumber one of the subnets (thx @Ron).
If each network connected to a local router is just a single subnet then that’s all there is to it (otherwise add more static routes as required).
If you can’t use additional, routed ports on the routers, you can add a router or L3 switch in between, connected to both networks, and add a static route to the remote network on each WAN router with the new router as gateway.
routing – Networking across campus with multiple ISPs
