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.
Tat
Edited, Jan 11th 2013 11:18am by tatankaseventh
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Tatankaa (71 Inquis)/Urkell (21 Conj)/ Seanthornton (21 Pally), <Inisfree> Freeport server
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Anaceup Mysleeves, 85 Erudite Mage
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<Inisfree>, Tunare (Seventh Hammer!)