Can you have a triangle with two right angles?

On a flat Euclidean plane, no. A triangle cannot have two right angles because the three angles must add to: $$180^\circ.$$ Two right angles already add to: $$90^\circ+90^\circ=180^\circ,$$ which would leave $0^\circ$ for the third angle. That would not form a triangle. On a curved surface, yes. The classic example is a sphere. Start at the North Pole, walk down one meridian to the equator, walk along the equator for a quarter turn, then walk back to the North Pole along another meridian. That forms a spherical triangle with: - a right angle at one point on the equator, - another right angle at the other point on the equator, - and a third angle at the North Pole. So the answer depends on the geometry. Flat triangles obey the Euclidean angle sum. Curved surfaces can have triangles whose angles add to more than $180^\circ$.