Wowzers!! Here's me with my poky 2 screen flight sim setup
Synergy is for multiple PCs. I assume you've already tried to adjust the monitor position under Screen Resolution in Windows? Other than that I don't have any experience with custom multimonitor products...
FSX is horribly, horribly, HORRIBLY buggy.
If that is the only problem you are having with it then you are exceptionally blessed and possibly the Messiah. No kidding.