While in the mortal world, the Demons have no way to communicate mentally. That would require magic. The Succubus could establish this magic link between herself and a demon or between two demons, if she wanted to.<br/><br/>However, the Demons are able to speak abyssal, which will come out as gibberish to all but the very learned mortal, so they could talk without letting the mortals know what they talk about.<br/><br/>As for the Demon pact, I'm not going to provide any rules for interacting with Witches that you have no pact with. Of course you can ask the witch you are partnered with if she could charm the witch in question. ;)