Ah, OK. Well in that case you could potentially create a failover gateway group with the two WANs. But I could imagine it would get stuck on the L2TP WAN. You would need to configure the L2TP WAN to be dial-on-demand and disable monitoring etc so it closed the link. But That would take far too long. Also I could imagine the PPPoE monitoring might fail until the L2TP disconnects.
It seems like you could script it though. Probably take a bit of testing to find the right rc calls to work with those.
Steve