i crossposted with @greg , so I’m saying the same thing here that he did, I’m just going to leave my post up because it has the screenshots and the link. 
It can definitely be done with core.
If you’re looking for a simpler way to do it, you can just use modes. Smartthings modes are different from the SHM arm states. See the following thread first, to make sure that you’re comfortable with the differences.
So then it’s easy. If the mode changes to Away when you have both left, just have the routine that automatically changes the arm state to disarmed not run when the mode is away. You can set this under “additional settings.” In the routine.
