^ What he said.
If you want a "backup" VPN server that's routed, it would have to assume the role of default gateway. With pfSense you can use CARP for multiple firewalls, but if your other firewall isn't pfSense, you probably can't do that.
If you use bridged VPN, it shouldn't matter because the clients will get an address on your LAN, so no worries about routing.