Marsupials are found in north america and australia, but they would not have been able to swim across the ocean. according to scientists, how did this group of organisms come to live on two separate continents?

The answer is continental drift. 

Continental drift is the geological theory that explains how continents move through Earth's surface. All continents were once connected as one supercontinent Pangea. On that supercontinent, populations of one species lived together in one location. But after continents spread apart, these populations were separated on different continents. So, all marsupials lived on Pangea. After continental drift, some populations remained in Australia, and other remained in North America.