After I registered one account with supercell ID, I can no longer load other accounts on same device.
I first disconnected current game account (the one registered with Supercell ID), then went to iPad Settings, login to different Game Center account, then launched Boom Beach again, it prompted for "Log In with Supercell ID" or "Play without Supercell ID", chose "Play without Supercell ID", it then prompted to load the Game Center account, clicked Load, went right back to "Log In with Supercell ID" or "Play without Supercell ID" screen, clicked "Play Without Supercell ID" again, it loaded a new game.
Now I can play only one account on this device. HELP!!!!!!