The behaviour for running running two or more instances doesn't change at all. You simply run mirc.exe again, and you have two instances running.
If you're talking about running two different exe files in different folders, it is clearly described in starbucks' post as well as the
versions.txt file in mirc.hlp. You simply put the mirc.ini file in the same directory as your second mIRC and mIRC will use that ini file instead of AppData's. You can also read /help Command Line to see how you can specify an mirc.ini file or mirc directory via the command line (or program shortcut), which is ALSO clearly described in Pivo's post.
I'm really not sure how people are still utterly confused on this issue. This information is literally all over the forums, likely on the google too ("mirc appdata" gets enough hits).
On a sidenote I should point out that it really hasn't even been necessary or all that intelligent to run two/more instances of mIRC ever since it got multi-server support eons ago. There's simply no real reason to need two clients running; you can do everything from inside of one single client and have been for the longest time.
