How do I avoid merging some bookmark folders

I'm using Xmarks on multiple computers, but need to keep some bookmark folders (notably the toolbars) separate on the different machines. I assumed that de-selecting those bookmarks in all of the profiles would prevent this, but that seems to have no effect (indeed, I can't tell that it does anything at all... except to increase odds of bad merges in which multiple copies of the same bookmark end up in the toolbar folder.

Unlike some of the "duplicate entry" threads, this does not appear to be related to Firefox behavior -- as long as I'm working exclusively for several days on the same machine, all is well, even if I synchronize Xmarks multiple times, close and open Firefox, reboot, etc. I've also learned (the hard way) to synchronize one machine before opening Firefox on another and to immediately synchronize the second machine, etc. -- your synchronization process, understandably, seems to have trouble keeping track of "latest" otherwise. But, as soon as I synchronize the second machine and then the first, the "bookmarks toolbar" folders get completely intermixed. Isn't this what having multiple sync profiles is supposed to prevent? Suggestions?

(I am not using Xmarks with IE, so that is not a source of the problem. And this has persisted both before and after the Foxmarks->Xmarks transition.)