No, a single server cannot have two license keys for the same matter (e.g. CALS = client licenses). You'll have to replace one license key by the new one and reactivate.
In theory, if your client buys additional licenses (for exampel additional user CALS), then they should have been added by Qlik to the original license key. And a simple on-line update of the license configuration would have increased the number of CALs. I do not understand why additional licenses are supplied using a new license key...
Note that a full Publisher has a different license key than the QlikView server. For that matter, the QMC offers a separate tab to enter the Publiusher license key.