I guess the real question is whether they should be stronger than the larger wyvern.
Hmm, I don't fancy making them stronger than a Dragonne or Wyvern. It would seem odd. (Although, if I was rewriting the monster SRD I'd make Dragonnes and Wyverns stronger - surely they should be at least as strong as a Str 21 Lion or a Str 23 Tiger?)
Anyway, let's do the sums before we get carried away.
A Strength 19 Large quadruped has a light load up to 348 pounds, a 349-699 pound medium load, a 700-1050 heavy load, and can drag 5250 pounds.
A sis'thik weighs 200-250 pounds. If we allow for, say, 100 pounds of gear apiece that means two sis'thik riders could weigh as much as 700 pounds.
If it's just two skinny 200 lbs sis'thiks with weapons (2 scimitars, 2 longspears, 1 shortbow & 40 arrows) and saddles that's only for 84 pounds of gear, for a total weight of 484 pounds.
Do we want the light load to be about that much, or do we want the Xytar to be somewhat encumbered if it has two riders? The SRD Heavy Warhorse's light load is the weight of a heavily armoured human, or 300 pounds.
Hmm... I think I'd just say Xytars have a carrying capacity multiple of ×4 instead of the ×4 that a quadruped of their size has.
Carrying Capacity
Due to their sturdy hexapedal build, xytars can carry heavier loads than quadrupeds can. Their carrying capacity is 4 times that of a Medium biped with their Strength score. A typical xytar can carry a light load of up to 464 pounds, a medium load of 465-932 pound, a heavy load of 933-1400 pounds, and can drag 7000 pounds.
That looks about right, don't you think?
