I think at this point it might be helpful to taboo "reality". I would rewrite your statement as something like this:

If my senses are transmitting perceptions that appear to be the result of my interaction with another agent, then, assuming that the agent does indeed have the ability to operate independently of myself, that agent and I share at least one communication channel that is common to both of us.

That sentence is unwieldy and, frankly, boring, but it does avoid making unwarranted assertions.

That said, going into this much detail is probably not useful. If you and your interlocutor can't even agree on whether both of you exist, there probably isn't much room for productive dialogue.