Hi Stan
Knowing your cactus are Copiapoa will help. I am going to assume they are both round, solitary cactus. I have never heard them refered to as turberous cactus before.
Try this link:
http://kadasgarden.com/Cpottin...
The difference between a round pot and a square pot is that a square pot is a round pot with corners. So a square 4-inch diameter pot will have more volume (64 cu. in.) than a round 4-inch diameter pot (50.2 cu. in.).
As you are putting two round cacti into one square pot, the diagonal of the new pot needs to be big enough to accommodate both of them. Maybe you should look for a shallow rectangular pot.
Daisy
