I'm sure there's an entire logistics problem around this. Do we know if the two doses are exactly the same? What about the interaction or effectiveness of the first and second doses being from different vaccines? How critical is the timing of the second dose, exactly 21 days, is 15 good enough, how about 30 or 45, or longer?
Depending on lots of answers, I can see where it would be easier, have higher quality outcomes, and less waste to require both from the same place. They could make both appointments at the same time, have inventory delivered in the correct matched quantity, of the matched product, and perform follow-up for missed second appointments fast enough to still be useful.