You can combine two license files when all the following conditions are true:
The number of SERVER lines in each file is the same.
The hostid field of each SERVER line in one file exactly matches the hostid field of each SERVER line in the other file.
The hardware key number in each file matches the hardware key plugged into your license manager machine.
Use the Update License utility to combine the license files:
On the license manager server:
Open the Update License utility located at Start > Programs > ArcGIS > License Manager > Update License.
Follow the instructions on the Update License dialogs to add the keycodes of one license file into the license file on the license manager server.
After the Update License utility makes the license changes it will perform a reread of your updated license file. Your license manager will be ready for use. You do not need to start, stop or reread your file manually.
Some possible reasons license files may not be compatible:
License files are set up to run on different server nodes, so hostids are different.
One file is set up for single server (has only one SERVER line), the other is set up for redundant servers (has multiple SERVER lines).
One vendor uses a custom hostid algorithm, so the hostids on the SERVER lines are different, even though the files are for the same machine.