They don't stack, as such - however, they don't need to. They're entirely different effects and abilities, and if both apply, then both deal damage. If you have imbued your weapon with psychic strike and use it to deliver a sneak attack, you deal the extra damage for both.

There's no text addressing "stacking" or the like because it's not an issue. It's damage from different sources. If there were a bit saying, specifically, "you cannot deal psychic strike damage and sneak attack damage on the same attack", then... well, then you should probably ignore it.

shadow_archmagi's answer was flippant (and a bit useless). It is true, however, that soulknives are pretty suboptimal, but you can probably survive playing one, unless you're running with DMM clerics and non-evocation wizards.