Hi,
I have a similiar configuration and have reconfigured my OpenVPN a few days ago according to this document : https://doc.pfsense.org/index.php/Multi-WAN_OpenVPN.
It seems to be working. If I disconnect my first WAN connection on the OpenVPN Server side, the Client reconnects after a short time via the second link.
If the first link is back up, when I disconnect the second link, the client switches back to the first link.
With this configuration the client side won't reconnect automaticaly if the primary gets back online again..it stays on the second connection as long as it is available or until you reconnect the client side, but this is exactly how I want it to work.