It is my understanding that FTM sync is a one to one relationship.If you set up a tree to sync with your laptop, and with online tree "?Jones?" then you can only sync "?Jones?" with that computer.If you try to sync with the desktop it will fail.
Please read these forums there are some people who have found a work around, but it is a little more complicated.
As my family tree is still a work in progress, I do not have an online tree.