The only problem is that you have identified the participants by their
names. But if they are anonymous to each other, they don't know each
other's names. So neither one can learn the anonymous mailbox of the
other from his name.
What I gather you mean is that there are two anonymous parties, both
communicating in some kind of electronic forum, and over time they
decide they want to communicate with each other. Perhaps they notice
that both have a shared interest in explosives and beautiful women.
So they proceed to send email, and perhaps proceed to discuss subversive
plans.
This is no different than any other kind of on-line meeting, except that
the two parties are anonymous. In this scenario, you're right that if
one is captured, he can't reveal the identity of the other.
However, there are some problems. With only online communication, you
can't be quite as sure of the sincerity of your correspondent as you would
be if you met face to face. If your conversation progresses to the point
where a meeting is necessary, that will be risky.
Also, with anonymous communication it may not be possible to get the
public keys meaningfully certified by third parties. So a "man in the
middle" attack may be more practical with this kind of communication.
Hal