On 1st router, setup the server config for openvpn. Create some user on linux so you can generate user config/profile. On 2nd router, import config/profile (if that is also Turris), you should use provided template (as is, just to have it generated in /etc/config/openvpn) , edit that file using generated config file from 1st router (not sure if you can use inline keys and certs, maybe it will be necessary to dump them to individual files). Turris also have option to just source external config and use it in uci/luci context.
Start server on 1st router, start client on 2nd router.
If the router is actually in same lan, you have to take care of routing or/and ensure your topology setup will work.
If there is already running openvpn server (on 2nd router) you have to reverse whole configuration . In that case do not setup openvpn server on 1st router, just generate client config on 2nd and migrate it to 1st.
Aside, in normal situation i would setup vpn on main router, disable it on second and mount the NAS from 2nd to 1st router. (just wondering, why having nas within vpn inside the lan network? or that vpn is going via internet ?? – if so, the whole setup will be a bit complicated)