Painter doesn't have that capabiltiy as PS does. There is a workaround way of doing it but it's a little more involved. It's explained HERE by Jinny.
Also you will have to drag the program across both your monitors as Painter does not allow document windows to float outside the main program workspace - only palettes have the ability to do that.
Win 7 64 bit, 24gb RAM, i7 x980 (6core), 2x GeForce GTX 460
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