Get the XL twin beds or you will have uncomfortable guests. I have one of my rooms set up this way. I have the twin bed frames and the "doubler" came from Innstyle. It's a sherpa pad with a strap that goes around the two mattresses.
I keep twin mattress pads on the beds, add the doubler and another THICK alternative down mattress pad on top. It is so comfortable, people don't know it's two twins.
This room has been very versatile for me. It's a great room for siblings, parent and child, friends and some couples have requested it as well.
My headboards are mounted to the wall. I just pull the beds apart about a foot and move the side tables a little. The one thing to remember is to get two XL twin bed skirts so you don't have to change them every time you change the bed.