If you are setting CPU Affinities, the Instance Balancer feature is designed to give same-named instances distinct, dynamically managed affinities.
Otherwise, you can potentially accomplish this with rules managed via the config dialogs under 'Options'.
If you have any control over the command line for the instances, then you can add a unique identifier there to match an instance. For example, if you add 'firstinstance', then you'd create a rule with the match phrase '*firstinstance*'.
Similarly, if the first instance launches the others, then you can match only the children by specifying the parent. For instance, if the process is 'something.exe', then:
/^(*,){5}something.exe.*,/
Read more about advanced process match strings here.