MikeJapan and H3lix have been putting up Double Killers on DJapes Forum. These consist of two layouts with very large cages which have the same solution, neither of which is solvable alone {without trial and error} but which can be solved jointly.
I have really enjoyed them and intend to put a few up in KiMo form over the next few weeks.
To start with, I have put up H3lixs Double Interdependent Insane Killer #2 as a KiMo. It is only slightly harder in KiMo form - so still in the mid-range of difficulty.