We are on dangerous grounds here, and this has to be clarified.
A baby stay is not a load bearing structure, it is there to support the mast, not to carry a sail of any sort. It is very close to the mast, about one quarter of the J measurement and it is a permanent fixture. It could run in any angle to the fore stay.
The stay sail or cutter sail is carried by a purpose buit stay with load bearing considerations. Hence the running back stays to counter it's loads. It must be in accordance with the rest of the rig's load bearing requirements. It is usually attached to the fore deck at half or forward of the J measurement. It usually runs parallel to the fore stay.
The double or twin fore stay, which is used for twin head sails instead of a spinnaker down wind, with the main sail lovered, could be like on Selkies picture fore and aft or athwartship.
The rig on the picture is very old school and l am not sure, it was bult like as it is now. I guess, it was a double fore stay later modified for a furler or a fore stay was added to the rig for some reason. It is a bit unusual. One would not be surprised if it would damage the sail on the furler.
The rig with a load bearing inner stay is not a sloop. It is a cutter. The sloop or bermudan rig was designed to carry a genoa while the cutter rigs were designed for a high clewed yankee, to clear the inner fore stay without removing or furling the fore sail. The genoa and the cutter sail is certainly an arrangement which is seldom used in conjunction. Certainly not to be used on narrow waterways, thus, makings tacking rather difficult.
In the modern world yankees are rear as hen's teeth. It is a typical cruising arrangement seldom used today.
If one is familiar with the old cruising bible of Eric C Hiscock's 'Voyaging under sail' one should see the difference. Bernard Moitessier's books are also a very good source for this kind of information.
A twin head sail:
A cutter rig:
The twins explained: