Hi Jonny,
Your explanation could use some clarification, what I understand is that you have two subnets and you want them both to be accessable for OpenVPN clients.
If that is the case I guess you can just use the "push-route" option like so:
push "route 192.168.1.0 255.255.255.0"
You wouldn't have to map ports although you will need to create some firewall rules on the OpenVPN interface to allow the traffic you require.