Combining crop and FF bodies: impressions sought from users of both systems

I have been considered getting a FF body for sometime now, with the idea that it would complement very nicely my 40D. At this point, I am rather keen in collecting impressions from users of both systems, i.e. those of you that regularly bring a crop and a FF body to the field and share lenses between them.

My view is that, at least in theory, a combo of, say, 40D and 5DMII would afford the best of both worlds in terms of speed, reach, definition and sensitivity. Would you say this is a fair take? Can you give some examples of situations in which having the two bodies allowed you to cover greater photographic opportunities?

I would also be interested in hearing about lens combos that have worked best for you when carrying both bodies in a defined situation, such as a trip or an assigment of some sort. I have myself some thoughts about which lenses would afford greatest flexibility when swapped back and forth between crop and FF bodies, but would rather have the views from those of you with real hands-on experience on this.

I guess I could try harder and define these questions a bit more, but I think I will start with this for now and see what it comes back. Thanks vey much in advance!

I struggled initially with the idea of "serving two masters" but eventually settled on this:

FF = Art .. APS-C = Action

I'll generally stick with wider angles (17-40 or 24-70) on my 5D for landscapes or "walking around" and will lean much more toward telephoto (300 2.8 + TCs or 100-400) on my 40D. In an odd twist, I'm finding that I really, really like primes on my 1DMkIII (85 1.8 and 50 1.4)