My understanding is that multi-wan is not possible on IPFire-2.x without significant reworking of the code. The devs will not look at doing that, as that would divert them from working on IPFire-3.x and supporting IPFire-2.x from security issues and bugs to existing capabilities.
From my understanding IPFire-3.x should be able to do what you want as it is being designed to be able to have as many interfaces and zones as you want to have and to use them in whatever ways you want, such as vlans, bridging, bonding etc.