Hellooooo!! Congrats for ur new pet. PD are awesome. If both are 9 weeks old there is not going to be any problem. They are babies and at that age, they dont feel like a stranger is in its territory. However, the way of putting 2 prairie dog together is as follow:
They have to be in diferent cages, so the one that is already in the house can go an smell the new one. Cages have to be in diferent rooms. If they stay some hours smelling one each other and the one in the house is OK, doesnt feel agressive, scarre or similar (you will know by the tail) you can move them to a neutral place in your house. Meaning, a place where they have not been before. If you put them together in the House one territory, fighthing and problems can arise. IF in the neutral place they are OK together, you have to repeat this 2 more days and in the 3rd, you put both cages in the same territory. They can share it but for sleeping, differente cages. After a while they will start sharing coges by themselves. However, all this is teorothy. Very usefull but teorothy. Each PD is diferrent and you never know what will happen. Currently Ihave 3 and experience with each of them had been diferent. Could you please tell me the cage type you have for them? This is very important too.
By they way, I work in a PD forum, one of the most important in europe. It is in spanish but if you are interested in see pic, cages, or participate, let me know and I send you the URL.
Regards and please, let me know how they go