Thanks very much to you both, Richard and Steve. But alas, I'm on a Mac. Should have said that, I guess. Unless there's a CoreAudio equivalent I guess I'm out of luck on this one...short of doing something monstrous like chaining my 2-channel pre through my 8-channel and using those six extra channels for other mikes. Something tells me that would be asking for trouble, though I might try it just to see.
Just to follow up in case useful to others, I was able to use balanced connections to bring the Line Out of the two-channel Scarlett in as INST inputs on the Saffire without obvious distortion or gain blowout (Focusrite suggested doing this as well). I am keeping the levels for mic -> Scarlett hotter than the levels from Scarlett -> Saffire, so most of the gain is being applied at the first pre in the chain, with modest gain addition in the second pre. It gets me a pretty warm, good sound. And now I can add a third mike using a free channel of the Saffire, which was where all this started.