It can be a bit confusing when trying to measure your gun length, generally speaking band guns are measured from the muzzle to the trigger housing but sometimes from the band hole to the trigger housing. Pneumatic guns (air guns) are measured as the total length excluding spear.
So a 90cm air gun is 90cm long, a 90 cm band gun could be 100cm long!
Regarding twin bands, this does not always offer more power or accuracy!
If you fit twin 16mm bands to a light weight, low mass spear gun you might get similar power to fitting a single 20mm band. Loading twins takes twice as long but obviously 16mm is a lot easier to load than 20mm!
However it is very easy to overpower a band gun to the decrement of accuracy & safety!
High mass guns (wood for instance ) with robust trigger units are very often fitted with twins, but 28mm barrelled alloy or carbon guns are normally supplied with a single 18 - 20mm band.
It is interesting to note that even now with lots of high power guns available, competition target shooting (popular on the continent) competitors prefer to use 1100cm single band guns in the open class!
So for more range and power buy a longer gun, a gun manufactured & designed to take multi bands (woddie or high mass carbon) or a pneumatic.
However you can tune your gun to get the best from it, quality thin spear, thin running line with no knots, 18 0r even 20mm power band with dynema wishbone & probably best of all a swimming pool to practice in!
For a short gun like a 90cm, lots of power & virtually no recoil, you cant beat an air gun.