Each license has a maximum of one activation enabled in default, i.e. the key can only be used on one device.
To increase activations, simply generate a new key with an increased number of activations on the server in the user's account in the section Policies> Activation keys.
You will then need to activate the license on both devices with a new key. Activation on each station will take one license.
To deactivate an existing license and recycle it, open the license manager in the administration console (Help> Licenses) and press Manage license. A web browser will open, where you select Activation date> All, select the license type (Outlook Sync or Desktop Client) and then search.
You will see a list of activated licenses. Deactivate a license that you are unable to activate, to release the license. You can then use the activation key again.