What polygon is formed when 2 equilateral triangles that are congruent and share a side

You form a rhombus.

Draw an equilateral triangle, and then mirror it around its base. All the sides are clearly equal, because they are all sides of the same equilateral triangle.

Moreover, the top and bottom angle are 60°, because they are angles of an equilateral triangle, while the left and right angles are 120, because they're 60+60 angles.