Otherwise known as a 'Binio'.
I was reading through my 1970 published copy of David Sear's
Roman Coins and Their values.
I noted that the earliest listing in the book for a double aureus is under Caracalla, who also introduced the antoninianus. In Sear's book, Caracalla on the double aureii is noted as wearing a radiate crown, as with the antoninianus.
Another reference I have is an auction catalogue for the disposal of the coin collection of the Metropolitan Musuem of Art, (NY) in 1972. This catalogue has ALL 347 aureus lots illustrated.
The weights for the aureii issued for Caracalla in this catalogue range from 7.06 to 7.59 grammes (9 coins).
I assume that the double aureus of Caracalla must weigh either around 10.6 grammes (1.5 times the aureus), or 14.5 grammes (2 times the aureus).
Not in all of my experience with ancients, have I seen a picture of a Double Aureus of the Severans, or have I been able to find any confirmation of their weight.
As I see it, I may have set a tough task, or maybe someone is lucky enough to have the information and a picture.