It's a little tricky, but you climb them like a ladder. The key is opening the top hammock wide before climbing into it, so you don't roll out. Also helps if your hang angle isn't too obtuse, which would make it easier to roll out of.

Basically if you keep them close enough in height relative to each other, and you use a normal proper hang angle, you can climb right up and in.

That said, my wife won't do it, doesn't trust herself to not fall. So if you feel uncomfortable, definitely don't try it.