my question is can I connect the 2nd vpn router to the 2nd Eth port on the NAS and thus access files from both networks?
You can connect to both networks if they are different subnets. But in the uplink direction the NAS could use either interface to send files to an off-net address.
You can prevent that by manually configuring the current NIC to use the second router IP for the router/gateway field. That might not be quite enough - you might also find that the ordering matters (e.g., you might need to swap the NICs).
