You can lock a single feature to multiple machines (license servers and/or clients) by using multiple <CLIENT_HOSTID> and <SERVER_HOSTID> tags. For each machine, you can lock the feature to multiple items (ethernet HostID, custom HostID, etc.). For example, you can make a local license that works on multiple specified client machines or a floating license that works on multiple servers.
For example, to create a node-locked local license that will work on two known systems, you can lock against multiple, different items on each host in the following format:
<CLIENT_HOSTID>
<SETTING ETHERNET="123..."/>
<SETTING HOSTNAME="ALPHA1"/>
</CLIENT_HOSTID>
<CLIENT_HOSTID>
<SETTING CUSTOM="ABCDEF..."/>
<SETTING USERNAME="joe user"/>
</CLIENT_HOSTID>
This example locks against ethernet+hostname for system 1, and username+custom on system 2.
If you make the license counted and specify both <CLIENT_HOSTID> and <SERVER_HOSTID> tags, the license can be hosted only on a specific license server machine offering licenses to a known client. Effectively, this is double system locking.
Another example is to allow any license server to host the license by excluding the <SERVER_HOSTID> tag, and allow only certain clients to use the license from the server by specifying multiple <CLIENT_HOSTID> tags.