One key will work in two machines.
if you want to move one machine to another then deactivate the key and use it in another machine.
As per Tableau EULA - you are allowed to use the Same product key on 2 instances - i.e. Workstation & Laptop as a ideal case. I would also read the Master Agreement if you are purchasing the license from an approved Distributor as Tableau does not directly sell licenses.
Refer to below link to read up on it:
"2.2 Tableau Desktop (“Desktop Software”): If you purchased or received a license to Desktop Software, the total count of Authorized Users enabled to use such Desktop Software must not exceed the number of licenses purchased on the applicable Ordering Document(s) or if there is no Ordering Document, as otherwise designated by Tableau. For each such license you may install, via the Product Key, one copy of the Desktop Software on one primary computer and a second copy on a secondary computer for each Authorized User."