Sorry, long delay between writing and posting because my connection went flaky last night. Anticipating ninjaing.[...]plus the difficulty of defending such a long connection between the two outposts.
The way I see
that point is that if something gets into my surface-entrance (i.e. within the central keep's stairwell, which is well within the outer defensive wall) then there's a lot of death and destruction going on in the easily-travelled innards that I dig out immediately sub-surface (and/or build on top, skywards). I already make precautions against invasions from magma-creatures, but if I have attacks from either there or above that actively break in, I always have the option of severing contact between the "two forts" (although I don't treat them as such), to save such dwarves as are currently in the safer place. At least until the food or other supplies begins to run out, if it wasn't the top end that survived, by which time I should be able to work something out.
Are you perhaps thinking about the link between the two being 'cavern-creature invaded' and swarming both directions? My current playing style involves no route-creation at all between fort and caverns. There's wells that could be flown up (perhaps, so far never have), as well as where the exploratory shafts dug down that punctured the cavern ceilings to reveal them, but I've got no walkable entry into them. And, if I did, I'd have one of my patented ditch-mediated protection system in place almost as quick as I went in there.
Of course, there's the problem of !!FUN!!, but that can hardly ever happen without being at least aware of the need to have planned for it.