Interesting - I thought the idea of Solent stay would be to have the inner stay fastened very close to the top of the mast (near but just below the forward stay) negating a need for an additional back stay?
We get so many varying terms for this apparatus! - baby stay, jackstay, slutter stay , etc. All of these are not what I meant.
There are two rigs I have never understood and put them down only to British ideas of racing-rules loopholes. One is a sloop but with a second headstay from an intermediate point (above the spreaders) to the stem. The other is the opposite, a stay from the masthead to a point on the deck maybe 2/3 back from the stem. I see no universal sailing value in either of these.
The second one (which you seem to be calling a ‘Solent stay’) makes coming about a nightmare. You will have to tack the head of the outer jib through a very narrow triangle at the top; and good luck with that!
The first one has the same problem but at the deck. In fact I’d say this one is worse.
Thus I expect these are only racing rigs, for when a crew can set and strike one of the sails and reset it on the other tack. I would strongly suspect that in either it’s the inner one they leave up - the only one of the two that can tack by itself.
Any shorthanded cruising application of either would have to be for very long legs in predictable, light air, such as setting the two wing-and-wing; and if that’s the case why not just go with two parallel standing headstays? - which has been done often - but which I’d never do.
My preference would be for a proper inner forestay, leading from some intermediate point above the spreaders to some corresponding point on the foredeck, essentially a 2/3 or 3/4 headsail. This can be removable, perhaps by use of a Hyfield lever, to permit tacking a large (outer) Genoa; but this isn’t necessary with a proper cutter setup. What will be necessary are proper running backstays.
On Cherubini 44s I have rigged both stays with Schaefer headsail furlers (3100 and 2100; and NO boom for the inner headsail!) which makes tacking, furling, and going wing-and-wing easier especially when shorthanded. Given my dad’s experience with C44s, this was the thinking behind the H37C.
My reasoning for adding an inner stay for Diana was something else entirely.