US Twin mattress - 39 in × 75 in
US Full mattress - 54 in × 75 in
US Queen mattress - 60 in × 80 in
US King mattress - 76 in × 80 in
US California King mattress - 72 in × 84 in
As mentioned in the question, combining two double mattresses will create a 108 inch wide by 75 inch long mattress. If you proceed with this plan, suggest building a platform bed frame for this non-standard size mattress arrangement so you get the width and length you require. The Ana-White.com website has many free building plans for bed frames which you can change to the specific dimensions needed.
The links below list steps for combining two mattresses, and the use of a bed bridge to fill the space between the mattresses.
http://homeguides.sfgate.com/double-up-t...
http://www.comforthouse.com/playhouses.h...
Bedding will be a problem although these websites which sell Ultra King mattresses also sell custom linens for oversized beds.
http://www.selectabed.com/ultra-king-siz...
http://mareebeds.com/
Two full size same color fitted full size sheets, with one king size flat sheet, to cover both, if using it as the same bed.
