How does the "Combo Group" employee OT group work?

We have a number of employee groups that have the same OT rules, but have a separate JDE OT rule for each employee group.

We would like to combine the employee groups into one OT rule to streamline the process.  When I started creating a new employee group, I noticed the option to create a "Combo Group" where I could select multiple employee groups... so I did that.  When testing this new group, it didn't pull in any OT.  However, when I ran the old groups separately, the individual rules worked properly.

My question is, how is a "Combo Group" intended to work?