I need help configuring SLI in xp pro.

Ok this is my situation.
I have two GTX 260s in my system and recently I decided to go tripple monitor setup with Flight Sim X and Rise of Flight. I tried to use SoftTH in vista but it didn't work so I dual booted XP Pro 64 with vista 64.
All went well at first and I was glady enjoying my games on tripple monitors. But the only reason I had gotten a second 260 was all because of SLI. But now, using SoftTH, I couldn't use SLI. But with little reserch, I found the following solution.
So I got me a 8400GS and plugged it into the system. And now here is my problem ( I reinstalled both windows by the way, and all drivers).
In vista, with two of my monitors on one of the 260s in SLI and the third on the 8400, I can turn on SLI and all will be ok. But with xp, its giving me a headache!!! Same monitor setup,,,if I turn on SLI the second monitor just goes black and only two will be up. In display settings, I can see all three monitors enabled but the second one just wont show anything on it!!!
Please if anyone has any clue how I can possibly work around this annoying issue.... gladly help me out here!!!!!!!!!!

to my knowledge SLI was never compatible with dual displays