If you swim butterfrog in competition (it's only permitted in masters swimming) you are allowed only one frog kick per arm pull. It used to be the case that the FINA rules had a loophole that allowed masters swimmers in Britain (and perhaps elsewhere) to use two frog kicks per stroke. Unfortunately that has now been changed and the FINA rule expressly forbids two frog kicks. You can use one frog kick and one dolphin kick, though, which seems illogical to me, but that's the present situation to the best of my knowledge.
There was some discussion recently on the USMS site about whether you can in fact swim butterfly almost entirely with dolphin kick alone, because the rules do not specify how many dolphin kicks you can take per arm stroke. Obviously it would be slower with just dolphin kicks but if like me you find it difficult to get the arms cleanly out of the water it could mean the diference between finishing and not finishing. My conclusion is that in theory one could swim legally with only one pull per length, but obviously that would not be very competitive and we are unlikely to see it in the Olympic Games.