A layout is specifically designed for one page. I have made maps before with legends on a separate page before and there are a few options you have. Since the release of ArcGIS Pro I have been making any multiple page maps in there since you can have as many layout pages as you want based off a single basemap. In one instance, I have twenty-plus layouts all based off of the same basemap stored in an ArcGIS Pro file.
Before ArcGIS Pro, I would simply use two MXDs. Just copy over your first MXD and then change the layout for the second. I had a map I made a few years ago that had three pages legend/index. Each page needed its own MXD.
If you really want two pages in a single MXD you will have to set the page layout to double whatever print size you are aiming for. Put your map on one half and your legend on the other half. When you print you will have two separate pages from one layout.