Multiple "Outside" interfaces and routing lookups

Doesn't sound like an optimal design but regardless, the Auto Last Hop feature will save you here but not if your firewalls are clustered and use VRRP things get a bit sticky.

You can find more information on ALH here: http://support.f5.com/kb/en-us/solutions/public/13000/800/sol13876.html.

From a design POV, could you not route the traffic from the additional f/w through the existing one perhaps? No idea of your set-up and even that doesn't sound great I guess. My worry is one day someone will disable ALH and things will go very wrong.