Eh, I think master is used (AFAIK) unproblematicly in other contexts like a master key, recording master, and master pattern. Converting it to "main" seems like a change or loss of meaning, but the problem may be that there is not really a consistent meaning across electronics usage to start with. I think "secondary" has some connotation of filling the same purpose or type as the primary, which doesn't really fit for m/s usage. Master/sheep is my most similar option that keeps the "m/s", but it feels awkward enough to draw attention to what it replaces. Could just do master (or main) and sub, where "sub" could mean substitute, subordinate, subscriber, [submissive,] etc. as needed.

I like sender/receiver