Am I correct in understanding this interaction?

The attacker picks the sequence in which multiple simultaneous effects occur. Therefore, they may levy the conditional stress from the cannon first, and then apply a second stress via Tactician.

Is that right?

No, but it works out in the attacker's favor anyway.

The Cannon triggers "When the attack hits..."

While Tactician triggers "After performing an attack...

or terminology similar to those.

What that means is that the Cannon effect occurs as soon as it is determined that the attack hits, in step 6 of the attack sequence, while Tactician does not trigger until after the attack if fully resolved.

So the target does get 2 stress tokens, but you have no choice in the matter, the Cannon must resolve first, then after the attack the Tactician.