On an older Dodge van the switch to roll up the windows on the driver and passenger sides work for, say a month, and then they don't, and then one side works, and then the other side, and then neither side. It's crazy. Most suggest a motor but that seems unlikely a cause on both doors at the same time. Ideas?
Intermittent open / short / bad ground in the wiring, flaky motors, or bad switches. You could pull a door panel and get busy with a multimeter when one is misbehaving. Assuming the circuit is fused, which it should be, you could probe at the fuse block and maybe localize a wiring fault to one side of the fuse.
You could always take the shotgun approach and start replacing parts on one side and see if you get lucky. Also, check any and all contacts / connectors, staticsan has a good point there.