It seems like Marco's solution is the most straightforward/easiest. Always go with the easiest solution:)
Since a layout is one page, yet you want to create something that is 1 layout but two pages (front and back when printed) the only way that I can think to do that would be to create a 2-page map series, meaning that you'd have to create an index layer, potentially with a dummy index feature for the second page... and then you get the legend on the first page as well... er, see what I mean. Try Marco's suggestion:)