But if I see a rider who can stop and put both feet flat while on the saddle, my first intuition is "saddle too low." But not being able to touch is not an indication of "just right!"
I believe it is possible for nearly all riders to put some part of a foot on the ground when stopped with the saddle set correctly. Whether there will be enough foot contact for good stability without twisting or getting off the saddle, I can't predict. But why not just learn to dismount to a two-foot frame straddle when you stop? That strategy is guaranteed to be nearly as stable as standing without a bike.