I assume they look like double doors, though maybe not meeting in the middle. Could you fit a peg to the top of each door at the far end from the hinge, open the doors to the required angle, and then slip a Meccano-type strip over the pegs so as to connect the doors together?
Then, viewed from the top, you would have a rigid-ish trapezium, though if the door opening approached 90° it would get more like a rectangle, liable to collapse into a parallelogram in the breeze. To prevent that, put extra pegs at the hinge ends of the door tops, and use two Meccano strips in an X shape (but not fastened together) . Then you would have two triangles, which would be more rigid and permit one door to be open more than the other.
Andy