PNR means "passenger name record," but you might know it as "record locator" or "reservation number." It's a six-digit alphanumeric number.

So if you book one ticket with miles and one with cash, you'll have separate reservation numbers for each. This shouldn't be an issue if everything goes as planned, but among other things, if there are delays/cancellations, the airline won't necessarily know that the people on different reservation numbers are traveling together in order to rebook you together.