Question: Does "specify commands to run" group policy additive?
Answer: No, it doesn't. The type of this "Specify commands to run" group policy is hard coded in GP template that it is not possible to configure multiple "specify commands to run" group policy.
Example: In "default domain policy" specify a command to run, and then do the same for a computer OU (different command) that this computer's object lives in. As a result, the command configured under the computer OU will be executed and the other command will be cleared.