There's no way to match those two outgoing connections together without external help.

Good point. That does seem to be the crux of the dilemna.

Ideally, they'd just multiplex the messaging and file transfer on the same stream, but it doesn't appear that any current IM service does that, not even Jabber.

Yup. That seems to be the crux of the problem with the IM services.

You could probably figure out a way to actually get two outgoing TCP connections to join up with some deep magic, but I imagine that's outside the scope of your request, and even then might fail depending on how your firewall rewrites ports.

Ah, but if it could be figured out, and one could write such a utility, I think one would have a very popular little applet to sell. Hm. Wonder what that magic would be? Perhaps something that piggybacked on an existing IM service's connection.