I had the same issue. Now, I just use TierAutoInstall, but there is a way to merge your .cabs.
Well, its kinda complicated. I wrote a tutorial a while back that highlighted some basic tasks in WinCE cab manager:
http://forum.ppcgeeks.com/showthread...ce+cab+manager
I believe you can import cabs. Then, you'd need to merge all the files between different installers. If I have time later, I might write a tutorial on that. [assuming I can get it to work with large .cabs]
Good luck~