The SolidNetwork License(SNL) Manager can host multiple serials. They are just added one after another separated by a comma during the installation process of the SNL Manager on the server. This scenario is common. For example, one might have a SOLIDWORKS serial and a separate serial for PDM.
The limitation here is that the version of the SNL Manager has to be the same version of the licenses. For example, if a SOLIDWORKS license expired in 2019 and the PDM license is on current subscription through 2020, then both serials can be hosted on a 2019 SNL Manager but not a 2020 SNL Manager.
This means that the latest version of any product that can be served by the 2019 SNL Manager is 2019, despite the PDM license being entitled to 2020. In order to use PDM 2020, a 2020 version of the SNL Manager must be installed and activated with the PDM serial. This requires installing the 2020 SNL Manager on a different server because only 1 version can be installed on a server at a time.
This creates limitations with who can use what license and on what version, which is a good reason to maintain all licenses on active subscription. This way all network licenses can be active on a single instance of the SNL Manager.