Q.Transplanting of Bird of Paradise
Can I transplant 2 Bird of Paradise Plants, 16″ tall, in a 9″ dia. 9″ tall pot?
Or would it be better to leave or transplant into a larger pot?

Certified GKH Gardening Expert
That can be fine, but they like to be rootbound for quite some time before being transplanted, so when you get them there you may want to let them establish for awhile.
This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/flowers/bop/bird-of-paradise-as-a-houseplant.htm