Answer:
I forget about these theorems and such, but pretty sure this is the Hypotenuse - Leg congruence theorem.
Explanation:
As the name infers, you need the length hypotenuse and the length of one leg of each triangle. This is because you can use Pythagorean to find the last side, turning it into SSS congruence theorem. You also need them both to be right triangles for pythag to work.