Well, I have run VNC in Solaris from X or PC, so you could piggy-back it.
Exceed might even know how to do dual control/view desktops, if you dig deep enough.
Screen gives a telnet like login many screens, each on a different pseudo terminal. The code is open, so it might be something you can add.
You might create a process that listens for telnet clients on a high port, and runs one shell, copying any keys down and all output back up to all. It is a bit like popen but with two pipe fd, and poll or such to manage not blocking on either. I suppose you could script it with named pipes, in PERL or with a C listener like the u_inetd.c I posted somewhere here:
Is this case is normal when programming in socket?
However, you want a pseudo terminal for full capabilities, which is in the screen code, or you could open not a shell but an rsh localhost (rlogin) or ssh/ssh2 to create your terminal. I guess if you started the first process under a terminal, you can use that.