Larry,
you state: "What I have found is that you should "sync" with only one computer. In my case I use a laptop and a desktop and sync only with the laptop."

By "sync" do you mean "TreeSync" with your AMT on Ancestry?
If so, how do you have it set? Manual or Auto?
Have you seen any issues where DropBox and TreeSync are fighting each other to update?

I use Dropbox to keep one of my FTM Tree files in sync between my Laptop and my Desktop. I have another tree linked and synced to a AMT on Ancestry using FTM TreeSync. This all works great, but I have been leary to try Dropbox with TreeSync at the same time.

For the OP:
FTM 2012 with TreeSync only works one FTM file on a PC to one AMT on Ancestry. You can have multiple Trees synced but for now it only goes between PC and Ancestry. There was some early discussion about adding sync between two PCs, but nothing recently. I am not optimistic that we will see that anytime soon, since they are still recovering from the glitches at original release.