I am in the process of doing this conversion so I will illustrate my plans with a drawing.
The plan is to place 3/8ths plywood over the existing frames in order to provide support for the mattresses, which are turned 90 degrees.
Panel 1 supports a mattress which fits into the curved corner at top left. The other end of the mattress butts up to a headboard. The headboard is being made from the ply left over when I had pre-made sheets cut for the panels at my local DIY store.
Panels 2 and 3, which are hinged together and hinged to panel 1, support a mattress. They are cut and hinged so as to allow access to the storage and tambours below. When access to the storage is required you simply fold one mattress onto the other. Then lifting panel 3 up you fold it over onto the top mattress - the 12" panel allows for the thickness of 2 mattresses. I have trimmed panels 2 & 3 a little smaller to stop them catching the curtains when I fold them up. I also cut the panel tops to the storage area 2" back from the end of panel 1 to allow access to the storage area.
If you and your partner like to wrestle in bed I would recommend some additional support in the span between the twin bed frames. I plan to make two supports which are no more than a custom length leg with a sheet of that plywood again about 6-8" square screwed to the top to sit under panel 1 and equally under panels 2 & 3. They don't have to be fitted to the panels, they can free stand for easy removal.
This design requires a headboard to be shaped, trimmed and upholstered to fit the tapered space at the bottom of the illustration.
I have 4 sheets cut to 62"(the width of the bed) by 7.5". From these I will make a box section trimmed, padded and upholstered to fit into the curved corner. I have found a template for this: It is the removable armrest from the front Goucho sofa. I will make a box section which will be covered with 2" thick foam and then covered with some silk I got off eBay.
If you trim the headboard right you can make the whole design reversible so that you don't have to argue who sleeps on which side of the bed and who gets out first. I am having cushions made to sit on the low seats of the remaining twin bed frames. I will use the same silk to recover the material trim bands which follow the line of the twin beds, but I will have the space behind the position of the nightstand filled in to match.
The nightstand, by the way, has been put in the wardrobe as storage for my socks etc. Lol!
If you are very keen on a nightstand you could build a very low unit which would sit where either cushion goes. Personally, I purchased a small edged tray which will support the obligatory cups of tea or cocoa at bedtime. It will also support a large Jack Daniels if you don't necessarily go to bed to sleep! Lol!
The whole thing is very cheap to build and light: probably about 20lbs of wood, foam etc.
Hope you like it!