It can be acomplished through jass.
if GetLocalPlayer != Player(1) then
call ShowMultiboard(YourMultiboard, false)
endif
i just took that from the top of my head though, not sure if it is all syntax corrent, but you get the picture. Just make sure you don't actually create things with GetLocalPlayer - that will cause desync - you can only use it to show/hide things or to affect graphical stuff.