First of all, I'm not a rendering guru somy thoughts on the differences between bump and displacement maps may not be relevant.
So if you understand the difference a rendering engine is using them for, you'll realize that a bump map will only affect the way (and "imitate") the shadows cast on a rough surface. Therefore (if both maps are used), a bump map should include the finer surface details rather than big, elevation differences (think of "spanish" roof tiles for instance). The displacement map then should include these "big" surface differences instead. Certainly you could achieve everything combining the two maps and only using it for displacement but I suspect that it would considerably raise rendering time.