Nevermind dual light lances. Go with one heavy & one light - threaten at 10 /and/ 5 feet!
Yes, nothing explicit in the rules keeps you from cleaving off a lance charge.
However, a charge is a very limitted form of attack. One of many caveats placed on the charge is that you have make your attack as soon as the target is within your threatened area. That means that the only valid target for a charge is someone whom you have just moved just close enough to threaten. With a charging horse and a heavy lance, that prettymuch has to be someone in one of the 5 squared directly in front of the horse. Anyone elsewhere in your threatened area you'd have to have moved /past/, thus making them invalid targets for the charge.
If you go with the idea that the cleave is basicly the same attack as the one that dropped the first target, only those characters you could have charged can be targeted with cleave. Of course, by that token, you'd also get to hit them with /all/ the modifieres for the charge, including the double damage (yipe!).
I do rather like Forester's idea, allowing the cleave to go only in straight line, but allowing you to move before taking it - that puts an apropriate trade-off this particular use of it.
Another similar might be to allow you to cleave off a charge by continuing the straight line of the charge until you get a /new/ enemy into your threatened area for the first time. Thus you couldn't cleave into, say, adjacent enemies, but could cleave through ranks of them...