You can't, if I understand your question.
ONE PC to ONE Ancestry Member Tree (AMT).
You need to decide WHICH PC you are going to use to Sync your AMT and Which PC you will update your AMT Online.
For example: Use your Desk Top PC to sync with your AMT, then work on your AMT from your Laptop. When you return to your Desktop, the the Sync feature will download the changes you made from your Laptop to your AMT.
Does that help?