Two developers can connect to a session if the user accepts them both,
and then they can type at the same time.

debug-me will reject some keystrokes depending on what the other developer
is doing. This is probably a bit confusing, and it could be a way to
plausibly deny a bad action, making it look like it was caused by an
inaverdant mix up of two developers typing.

Better would be to only let one developer type at a time, and they have
to pass the typing stick to let the other developer type.

Problem: What if one developer has the baton and disconnects? It should
pass to the other developer, but disconnection does not currently
cause a protocol message.

Also, related problem, one developer has the baton and goes away,
still connected.

Seems that perhaps the user needs a way to switch control to another
developer.
