The only way I think this could work would be to first buy a headphone splitter (Y adapter) then buy two sets of bluetooth headphones, but ensure that you buy the type that has it's own bluetooth transmitter, connecting to the iPad via the headphone jack.
One problem I can think of being the splitter won't give a stereo signal, so each set of headphones will only get one side of the audio. To make sure they each get all the sound you can change the iPad output to mono in Settings - General - Accessibility. (won't be stereo, but they will at least hear all the sound)
Also this means you would have to suffer having two transmitter blocks hanging from the iPad.
Hope this helps, can't think of any reason why it wouldn't work.