This is an allusion to the creation of Adam in Genesis 2:7. The God who was mighty to make lifeless dirt (already shaped as a man) alive is more than mighty to do so again (as also foretold in Ezekiel 37:10). As a result these two dead witnesses stood on their feet.
11 But after the three and a half days a breath of life from God entered them, and they stood up on their feet, and great fear fell on those who saw them.