At the moment, I carry a dSLR kit, and my girlfriend makes the most of my compact. If she ever bought a dSLR of her own, I'd guess she'd have an 18-200 (or similar) for her camera, I'd carry my normal kit (two or three zooms, fisheye, flash, tripod) then we'd mix and match the lesser used bits and pieces as required.

Depending on shooting styles, you may want more duplicates like wide angle zooms, or flashguns, but I'd try and pool as much as possible!