1. No that would violate the game rules. ISBoxer does a lot of things but the biggest thing it does is let you control action in multiple windows (instances) from your main window. So, for example, you can play your warrior and have a hot key in that window which you click to have your cleric cast a heal on your warrior. No need to alt-tab back and forth., Consider the possibilities!
2. Generally, it's best to assign cores to instances if you're playing them all on one computer. I believe ISBoxer's website has info on this.
3. Google for info on this. I think ISBoxer is the best but there are competitors. Personally I use multiple computers but I have tried ISBoxer and it works very well. It's not free, however.
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Sippin 115 DRU **** Firiona Vie ****Agnarr
FV: 115 WAR ENC CLE MAG WIZ SHD SHM Master Alchemist ROG Master Tinkerer & Poison-Maker
Master Artisan (300+) * Baker * Brewer * Fletcher * Jeweler * Potter * Researcher * Smith * Tailor
Agnarr: 65 DRU ENC SHD MAG CLE ROG WIZ BRD WAR