Jonjon,
The two sites are supposed to establish VPN similar to lan to lan?
If so have you given a though to SVTI or SVTI-DVTI configuration?
It pretty much depends how it's configured and how much you're willing to change it. Starting with routing.
Multihomed site can run DVTI of a loopback and branch can run two SVTI tunnels to the DVTI (running routing protocol to choose one)
Marcin