If you hit someone with a sneak attack that has a touch spell on it would both instances of sneak attack go off or would just one?
When you use spellstrike, you replace the ordinary touch attack that you would get as part of the spell with a weapon attack. Since there is only one attack, sneak attack only applies once. Sneak attack requires a separate attack roll in order to deal its damage again.1
Note, however, that you can use spell combat to attack then also cast a spell. The attack can trigger sneak attack normally, of course. And then if that spell is a touch-attack spell, you can make another attack (with a weapon, or as a touch, as you like), and that would trigger sneak attack also. This works similarly to how two-weapon fighting does for a rogue; if all you really want is a way to fake Two-Weapon Fighting, the arcane mark cantrip is on the magus spell list and uses a touch attack—which allows you to use spell combat and then spellstrike at will.
(All of that assumes that each of these attacks qualifies for sneak attack—flanking is the easiest way to ensure that.)
- There is probably an exception to this somewhere, but it’s going to be almost-always true.