Failover LAN + ADSL Mikrotik

Dear good days

I tell them with a theme that is drying up my head, I have a branch optical fiber connected to the RED of the company to a Mikrotik RB750, but that fiber is unstable and falls, so we have placed a branch ADSL to connect via VPN (open VPN) to our network with Mikrotik RB750.

What I need to do is disable Failover interface I (PPPoE) while the fiber is running, since within the network I can not turn on VPN, and when to drop the PPPoE interface enable fiber and ovpn-out1 (VPN ) and vice versa when it’s redpor Fiber

Any suggestions, as I have read several of the Failover they are but none disables the interface of another

Thank you very much for your time

regards