I tried recently to box on the same PC, it didn't work out well. The only way I've seen it done effectively is to have multiple pc's. I do know you can maybe dual-box or higher on one pc but the software needed was a bit fishy through the windower program, and it wasn't worth the time investment in my opinion because FF14 is in beta here soon and right around the corner.

If you do find out how to make it happen let us know but it might be more time than its worth.