I think Steel answers this.
Here is what he says about topmast back stays.
THE BREAST BACK-STAY
has a single block turned into the lower end, with a throat and round seizing, through which the runner is reeved.
One end is made fast to the chain-plates, abreast the mast, with a half-hitch, and the end seized down. In the other end is spliced a double block, connected by its fall to a double block that is strapped with an eye, through which a span is reeved, that has an eye spliced in each end, by which it is lashed to the chain-plates.
AFTER-BACK-STAYS
are set up, the same as a shroud, to a small dead-eye in the after end of the channel.
SHIFTING-BACK-STAYS
are clenched round the topmast-head, and a thimble spliced in the lower end, to which is hooked a tackle, the lower block of which is hooked to an eye-bolt without-board, and frequently shifted from place to place.
BACK-STAYS
are extended to the channels on each side, and are to support the topmasts, and assist the shrouds, when the mast is strained by a weight of sail.
Here is part of my Investigator STD rigging plan.
Steel calls for 2 backstays (for a vessel like Investigator) and I showed them going to the after end of the channel.
However reading his description, it sounds like 1 could be a stay to the middle of the channel and 1 to the after part.
Most references show both to the aft part of the channel.
On a larger vessel 1 backstay and 2 after backstays. This is confirmed by the drawing in his book.
Regards
Allan