All mirc versions
/query nick1,nick2 will open up a single query window.
IRC protocol will mean that nick1 and nick 2 get the same messages.
Replies, however, open a new query window from the nick.
mIRC should do one of two things:
(1) Route messages from that nick back to the nick1,nick2 query
(2) disallow the comma with query windows and open two seperate windows (like what happens with /join #1,#2, even if that is a server init'd event)
This would mean that 'group' conversations like you see on MSN / partylines would be easier to do.