How many gallons? I can see no reason that 2 x 8" fish can't go in a 60 - 90 usg.
I think the formula is 2 x height and 1.5 x width with of course say 1" per 2 usg for Synos. This is very rough. You would get more response and help in the Other Catfish forum with the Syno phobes.