2 OpenVPN, 2 IP and failover

[hakkerman]

I have two different OpenVPN links to the same server.
One of links is indirect (because one of WAN's in MultiWAN is behind the NAT and the server is behind the NAT too - we have to use 3-rd party server in the case of fail).

Now our server accessible with 2 different IP's:
OpenVPN1 -> IP1
OpenVPN2 -> IP2
Usually we use only IP1, but when our primary ISP is down, we have to switch to IP2
I'd prefer to have one virtual ip or something similar - which will route VirtualIP to IP1 in general situation and to IP2 when our primary ISP will be down.

How to do this?