As for differences, you wont be finding much between 2 quality scopes in a bracket.
What can make a difference however are eyepieces. Spotting scopes are designed and intended to be just that, individual scopes.
Today just about all manufacturers have chosen to use zoom eyepieces as opposed to single power.
Ive personally yet to see a zoom eyepiece that delivers the same viewing qualities as a good single power lens.
The only one to my knowledge, that still offers single power as well as zoom eyepieces is Kowa.
Another potential problem can be alignment issues with zoom lenses. If one is set at say 20x, but the other is actually slightly more or less power when set at the same power level, user problems can occur. There can even be slight differences in fixed power lenses.
Many people are using good quality glasses that are at least slightly out of alignment and they don't even realize it.
The only way to have it corrected is a trip back to the manufacturer or a certified repair center.
On the other hand twin spotters can be aligned on location in minits with an allen wrench.
Actually some of the older scopes are still very much in demand for twin scope applications, and not all are unfriendly in a backpack.