XenoMC is a brand new MC program which works in the same way as NeoMC, but it is open source.
Simply run the program and then open as many Tibia clients as you desire, of any version -- this program will facilitate all of them.
You can download the program itself from the following page:
http://xenobot.net/downloads/XenoMC.rar
You can view the source code and get a checkout from this repository:
http://code.google.com/p/xenomc/source/browse/
Enjoy!