No one can help with whether your router is dual band, as you didn't mention the model. It doesn't really matter though, as splitting bands that way would limit single band clients from connecting to one of the networks. It can be done with virtual access points, even if it's single band. Take what is needed from here:
https://www.dd-wrt.com/wiki/index.php/Multiple_WLANs
Then add your VAP subnet to the policy based routing box on the openvpn setup page.