In the real world, if I was running a major global airline, & wanted to purchase both B737-700's & -800's at the same time, do you think Boeing would say no? Or if I wanted to purchase both A320's & A321's, do you think Airbus would say I could only have one type at a time? I don't think so. As long as the plane/s you want are in production, they'll happily sell you anything!
So why in this game can we not do the same thing?