While charging from the alternator, the different battery chemistries will happily co-exist, though they might be happier at slightly different charging voltages. It's when not charging that the problems will occur. If you have a contactor on each battery and only power the airplane from one or the other when the engine is not running or the alternator not charging, it should be OK.
You'll need active notification that the alternator has quit so you can open one or the other contactor. This might be interesting. My B&C voltage regulator flashes a light on the panel when the buss is below ~13v. The LiFe battery is normally above that so it wouldn't necessarily notify me right away and I would waste precious LiFe battery power trying to charge the lead acid battery. I'm sure there's a way, but I haven't been looking for it very hard. If I replace one of my PC680s with an EarthX, I'll probably do both, which when I think about it, still leaves me looking for a way to activate a warning at a higher voltage.
Ed Holyoke