If I can confuse the issue a little more, the UA description of gestalt characters specifically mentions sorcerer/wizard gestalt, which gets the spell progressions of both classes.
It could be argued that sneak attack is comparable, but then, if it stacks, a gestalt character could get a total number of sneak attack dice greater than that available to any normal single-classed character. No spellcasting gestalt gets you 6th level spells faster than they would come to any normal spellcaster. Gestalt isn't about being more powerful than higher level characters, but about being more versatile and more durable than other, non-gestalt characters of your own level.
I believe that sneak attack dice should overlap, not stack, for gestalt characters.