Doesn't even have to be in windowed mode, though swapping takes longer that way.
There are also third party programs (which don't violate the EULA) which support the management of more than one instance of a game running on the PC. Just do a search for boxing s/w for more details.
Edited, Jan 11th 2013 11:18am by tatankaseventh
Tatanka Wolfdancer, 106 Wood Elf Druid, 7 x 300+ Master Artisan, 7 maxed trophies :)
Anaceup Mysleeves, 105 Erudite Mage, 2 x 300 Master Artisan
Snookims Whinslow, 105 Erudite Enchanter, 2 x 300 Master Artisan
<Inisfree>, Tunare (Seventh Hammer!)
Tatankaa (95 Inquis)/Urkell (38 Conj)/ Seanthornton (24 Pally), <Inisfree> Freeport server (some other server now)