I am confused. You're trying to stay on the saddle while stopped, with one foot down? You would have to be an acrobat to accomplish that feat!

Hope I can be clear, but try to think of the sequence as unclip one foot as you approach a stop. Then, in one motion, lean your bike slightly in the direction of your unclipped foot, brake, and get off of the saddle. This should eventually happen quickly and smoothly. When you are stopped, you should be clipped in on one foot, with the unclipped foot supporting you on the ground. Probably, only your toe will be on the ground, and you will be straddling the top tube, out of the saddle. The clipped in foot needs to be ready to push off, so you can push yourself up, onto the saddle. It may take a bit or two to get your unclipped foot back into the pedal, but that happens with practice.
I know some new riders feel they need to unclip both feet before a stop, but this is not necessary. I agree with Oak. Practice with one foot, develop the muscle memory, and then start on the other foot.