I would think you could send for the two in one envelope, if you put a note in and make it clear. The two ladies live next door to each other, so I am quite sure they can come to some arrangement.
If you're sending an s.a.e., there's no need to include postage on your cheque. Only add the postage if you're not sending an s.a.e. If you want two or more copies of any of the quizzes, just make sure the envelope and postage will cover the increased size and weight. Both quizzes are on one A4 sheet, so two quizzes will go in a 9" x 4" envelope easily, and an ordinary 1st or 2nd class stamp should be enough.