There's a good chance you'll have to buy more then 1 girth to fit that range but it's hard to know what sizes you'd need without trying it on the horse. I ride anything from small welsh ponies (which use a 44 or 46) to giant selle francais (54ish) and anything and everything inbetween. I use my 48 the most.
There's a chance your saddles could fit both horses, height isn't the determining factor in whether or not your saddle will fit. I ride horses for a living which means I ride a pretty wide range and I'm certainly not going to buy a saddle for each horse I ride. I haven't had any issues having my saddle fit (no back problems) but I do own a bunch of different pads for those that are harder to fit.