DD could jump with both feet by about 20 months but I remember that she was the only one in our mother's group who could do it at the time. DS is 19 months and waaay off - still does the little tippy-toe thing you describe and 'trots' around the trampoline. Soooooo cute!

From my very brief research, this looks like a skill that is likely to be developing for most around age 2.