It's been said before and deserves being said again-
Cleave (and supreme cleave) should strike with both weapons when a player is Two-Weapon-Fighting. The odds of the off-hand weapon striking should be equal to the investment of Two-Weapon-Fighting feats. Thus if Greater Two-Weapon-Fighting feat is taken, Cleave attacks should have an 80% chance to strike with the offhand as well as the main hand.
If this is too difficult or time consuming to program for the developers, simply make it 100% chance offhand on cleave. Just get it done and be finished with it already. You developer fellas' have already accepted that Cleave was broken (took you long enough!) last patch by increasing the swing speed... lets finish the job and make it appropriate for TWF players.
If you agree with this suggestion, don't let this thread disappear like all the prior similar suggestions regarding cleave. Player desired game changes that slip onto the backburner go further ignored. This is a (seemingly) simple change that aught to be dealt with soon (rather then the typical turbine 2+years of waiting).