It's my understanding that dual headstays are close together and parallel; the solent stay originates at the masthead, but takes a steeper angle down to the
deck, diverging from the headstay, to attach at a point well behind the stem: more or less where a staysail attachment would be fitted. No
backstays are necessary, but the solent rig can be used effectively on fewer points of sail than can an ordinary (fore)stay sail.
Here's a picture of Jack Tyler's solent rig (the solent stay can be seen on the right, following the headstay but diverging from it on the way down):
The ability to size pictures would be great, guys…
Jack writes about the
installation and use of this rig (one of many very useful articles on the site).
md7a, with a double headstay, reducing canvass would work much as you imagined, but if you douse the
genoa in favor of a working jib, the breeze is freshening, and at some time someone is going to have to go forward to put that genny to
bed. The downhaul won't tie it to the pulpit by itself.
You'd have to figure out whether you wanted to install separate tracks/cars for the second sail, or try to change the sheet attachment between clews in the middle of the sail change, adjust the cars for the shorter sail foot, and campaign on.