There are a lot of theorem in whole Geometry, but as you need only 2, it could be anything from that like:
1) Theorem of Complementary angles: The two angles will be complementary to each other, if their sum would be equal to 90 degrees.
2) Parallel lines characteristic. - The Parallel lines are straight lines that are in the same plane and do not meet, no matter how far extended in either direction.
Hope this helps!