Hi TanyaBennett,
Thanks for flagging this. I can see how that'd be a bit confusing, especially with both pay runs on the same date and already sorted with STP. From the screenshot you sent through, there's a little red X icon showing up. That's usually MYOB's way of saying "hey somethings needs fixing here first." If you hover your mouse over that red X, it should give you a quick message explaining exactly what's causing the issue and what needs to be tweaked. Once that's sorted, you should be able to include the other employee in the super payment without any dramas.
Regards,
Sai