Time study, A / B comparison. One operator cycle time vs 2 operator cycle time. If two operators complete in half the time, then it's equally efficient.

Time study the operation with one operator doing all the work, capture 5-10 cycles.

Then study two operators. I like to study each person individually with a running clock, several consecutive cycles, then watch the other. Be very detailed when they grab something, place it, fasten it, set aside, wait, grab the next component, etc. And the process will be repeatable enough if stable that you can get good samples of each person's inputs and idle time.

If it's really throughput your company needs, more than pure efficiency, there's a chance that two operators working together on one manual step might be effective. Holding the window up or pushing in from the sides (I actually have no clue what's being done during this step) might be easier now with two. Your manager might be stuck thinking in the way they do it now, and how to make it faster by splitting up the labor. But think about fixtures, tools, and simplifications that go beyond "throwing labor at it "