... and if so how? tried the plug and play method .. no luck. What networking wizardry must be performed to achieve this task.
Outgoing connections are going to use whatever modem is specified by the Ethernet gateway address in the datalogger. It is nothing unique to a CR310. That is normal IPv4 behavior.
Rather than two cellular routers, I would suggest one dual SIM model. The RV50X is dual SIM. Forcing which SIM to use if both networks are available is tricky. Fail over if one network is not available is automatic.