A triangle can have two perpendicular sides. If two sides are perpendicular, the angle they form is a right angle. A triangle can have only one right angle.
A triangle cannot have two sides perpendicular to a third side. The sum of the three angles of a triangle is exactly 180 degrees. It must have three sides and three angles. Thus, it cannot have two angles whose sum comes to 180 degrees.
In a right triangle, two sides are always perpendicular. The measurements of these sides are used to calculate the area of a right triangle. In an acute or an obtuse triangle, no two sides are perpendicular.