It doesn't matter if Panorama is open on both machines. What matters is whether there is simultaneous activity on both machines. Panorama monitors mouse and keyboard activity. If Panorama is open, but there is no actually activity, it doesn't count. You can leave Panorama open in the background on a computer all month, but if you never use the keyboard or mouse with Panorama as the active application, it won't count at all.
However, Panorama keeps track of activity in 15 minute intervals. So if you worked 5 minutes on one machine, then 5 minutes on the second, then 5 minutes back on the first machine, Panorama would count that as activity on two computers. If you do that more than occasionally in a month, Panorama will count both computers in that month.