I bought these once - never again. All they did was chatter on my windshield. I threw them away after a month and bought some much better ones - Autozone brand.
Would I have to make 2 different purchases, for 2 different size wipers, for the same car?
The purchase limit is five total wipers. You should be fine purchasing different sizes in the same order.
When you go to the page, you can select the size and quantity and add to cart. As the previous poster mentioned, you can buy up to 5 (regardless of size) so you can mix and match. That is what I did. I bought 3 for my minivan and 2 for my sedan.