We have rear "twin" beds in our AS. However, they are not the same size as what is generally considered to be a twin size, ie, 42" wide by 75" long. Our twins are 35" wide by 78" long and feature a curved corner.
AS does sell bedding at Jackson Center, but I'm not going to spend $200 on a set of sheets for each bed.
Having picked up a 40-year old Elna Lotus sewing machine at an estate auction for $25, I decided to make my own sheets and mattress covers, all with the guidance of an old friend who does a lot of sewing & quilting as my wife has never learned or wanted to use a sewing machine.
For the sheets, I bought queen fitted sheets at Walmart - got the percale cotton sheets for about $20, and followed the instructions I found on the internet Sew Your Own Fitted Sheets
The results are pretty good first time around. For the next sheet I do, I'll take the time to properly lay it out and mark the darts instead of playing dressmaker.