If you want full connectivity on the second Rpi you need to run the routed service in the RPi with both connection to transform it in router. Then every computer in the network need to know the route to go to the other subnet.
There is other possibility if you not need full connectivity, for example using ssh tunnel to the first Rpi that end to the second. Or for http run a proxy on the Rpi with both connection.

This is not exactly your configuration but this can help to understand the router concept: