Thats a really weird issue, unfortunately i don't have any experience with dual displays on a hackintosh. If you're currently using injectors you could try some EFI strings instead.

Otherwise I found this thread which might be some help to you:

http://forum.insanelymac.com/index.php?showtopic=103549

Good luck!

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 Mac OS X 10.6.4 Retail || Intel Core2 Duo E8200 2.66GHz || Gigabyte G31M-ES2L || GeForce 6600 GT 128MB || Realtek ALC883 Audio || Realtek RTL8169 LAN || Samsung SpinPoint F1 1TB HD || Sony Dual Layer IDE DVD RW DW-D22A || Apple Aluminium Keyboard || Mighty Mouse

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