Apertures One — Darrell Bain

Eighteen year old Twins Jan and Jani discover the existence of alternate Earths in a most unusual way. The first exploring party from another alternity immediately recognize that twins Jan and Jani Jenkins have a powerful but unrealized talent for creating apertures into alternate worlds, but at the same time the twins suddenly realize they can spot an aperture and the person who formed it at a considerable distance, a talent they didn't even know they had. The explorers from the Pankan Empire on another Earth have never seen the like of the twins, especially after they learn to create apertures to other worlds. The Panks are determined to not only subdue our Earth but to capture or kill the twins before they can develop their great ability to a point where they could help America and her allies resist the invasion. Within days, following one narrow escape after another, the Twins keep learning more and more about their abilities.