How exactly does Allen's telepathy work?

So this season, multiple people can communicate at a the same time using telepathy and since Allen is the only telepath there, I'm assuming he is just being very accommodating to everyone including the viltrumites they are currently fighting. However, when Allen met Immortal, he could only communicate to one person at a time and Mark and Immortal couldn't communicate with each other. Have I missed or forgotten something that explains this?