Two angles are congruent if their measure have a sum of 90 degrees *

If two angles have a sum of 90 degrees, then they are complementary. If two angles are complementary, then they add up to 90 degrees. (If two angles have the same measure, then they are congruent angles. If two angles are complementary to the same angle or to congruent angles, then the two angles are congruent.

Yes, if two angles are each complementary( two angles are complementary if the sum of their measures is 90) to a third angle, then they're congruent to each other.