I wish I knew how to drag a link over here to a photo, but you'll need to suspend the second one above the first on some sort of legs and then devise a ramp that isn't too steep.
Attach legs to the small one like a table so it can stand inside of the larger enclosure. Cut a hole in the side of the small enclosure with a saw. Build a ramp with sides and connect it to the hole you just cut on one end and the floor of larger enclosure on the other.
Google "two story tortoise table" like I did. There are hundreds of pictures. Hopefully a few of them will be helpful.
If you have two of the same size. or nearly the same size, you could remove 1/4 section of the top table and place a ramp there going down to the second level. Maybe sacrifice the sleeping area of one of them to make room for the ramp.
This is something I've never done before.