1) If you are experienced and can use a hex editor and know how to create .reg files, you can do it yourself, this is however very risky and may not always work.
2) You can post here, or come onto irc.WorldGamers.Net ( #GuildWars ) and ask me niceley.
I have hex'd the GW.exe (will have to do it each time theres a new patch) and can now run 2 copies of guild wars, Im not putting the zip for manual download (IRC > DCC only) because of the risks and potential risks involved... I have a disclaimer in the read me should it appear on any websites but i use it, and the picture below should show that it works, I took it using a camera, instead of a Print Screen to prove theres no photo shopping trickery or anything...
If you would like to beable to do this, pop onto IRC, and ask me, I can also then offer help if you cant get it sorted.