If you want them just to listen then thats straightforward, just make a splitter box for the 1/4" plugs. OR
1/4? Stereo Plug to Two 1/4? Stereo Sockets Adaptor > Maplin. Go buy some cheap stereo headphones from Argos.
You could modify some David Clarke ear defenders which use the same cup and headband and fit your own speakers.
Problem with microphones is getting them all to balance otherwise one mic will rob the others of power if missmatched. This can happen even when you mix Dave Clarkes of different mics.