I’m not sure I understand. If you only put one foot down to the ground, won’t you be doing the step test just on one leg?
I think the test is designed by putting both feet to the ground and then alternating each leg for the step-up portion.
Also, it’s not designed to be comparable to an outside version. By completing the test as prescribed, you’ll have a better idea of where your fitness lies in the spectrum of other climbers that have completed the test.