It can be done, but you need to be careful with your process.
First you back up the file on your desktop computer, making sure you select the "Allow restored file to resume syncing" option:http://ftm.custhelp.com/app/answers/detail/a_id/4378
Then you take that backup (.FTMB) file to your laptop and restore it there. When prompted, DO allow it to restore the link to your Ancestry tree:http://ftm.custhelp.com/app/answers/detail/a_id/4379
Once you've done this, the FTM file on your desktop becomes unlinked & should not be used (in fact should be deleted/hidden/removed from list to prevent you from accidentally trying to use it).
Reverse the process when you get home.
Another option is to get an iPad, use the free Ancestry app. It will sync with your Ancestry tree and you can make limited changes off-line and sync when you have Internet connection. Those changes will then sync down to FTM when you get home.