twin backstay can place more strain on the foot of the mast whilst not having as good an effect on preventing the forestay from sagging when close hauled. It will also put more weight up the mast. I had twin backstay rig on my last boat and this was needed by the design due to the location of the deck
fittings. I would much prefer a single
backstay. If the change is due to a concern about strength / increase the size of the wire / or change to dyneema
are fitted purely to fractional rigs in order to balance the pull from the forestay when close hauled. They are fine on a well crewed yacht, but a pain in the @@@@@ when short handed as they need to be released and made up as you tack.
A lot of boats have a set of baby shrouds set up against the baby stay and fitted to the deck
astern of the mast in order to add stability / but that would have been part of the mast design.