Keep in mind birds are flock animals and tend to better in groups. A group of smaller species birds such as Finches, Budgies, & Cockatiels do well in aviaries. Birds also travel horizontally in flight, not vertically. Thus a cage of greater width will allow the smallest of birds to fly back and forth more readily. Birds need to flap their wings to keep them exercised just like humans. (Is that a clothes rack or a treadmill in the corner of the room?)
If you have a multiple small bird aviary, try to arrange perches, toys and accessories so there is some clear path enabling flight from one side to the other.
Larger species (Conures and larger) pose a different set of problems. Typically we recommend every bird have his or her own cage. You may have more birds than space and before to seek a more efficient use of birdie real estate. You may have a pair of words you, or nature is casing to breed.
In most cases, breeding pairs should share the same undivided cage so they can perform their parental duties together. Sometimes you feel two birds would be better off together to keep each other company. This can only be determined by trial and error. Peaceful coexistence outside the cage is no guarantee harmony will prevail one the cage door is closed. There are territorial issues, Alpha male issues and the general-irrational-behavior-of-parrots, issues.
This is where Partition Parrot Cages and Multi Story Bird Cages provide the best solutions. Partition cages allow two birds to be together in the same cage but kept mostly out of harms way from one another. If you have doubts as to whether one birds may be aggressive against the pother, don't rely solely on the partition. Feet can easily be attacked through partition bars. To avoid this you may want to introduce a sheet of Plexiglas attached to the partition. Plexi can usually be found at a home improvement stores. Some will cut to your dimensions. I drill small holes around the edge and attach the plexi to the divider with cable ties. If you want the divider to still slide out make sure you attach the plexi and cable ties so as to not interfere with the mechanical movement.
If you are simply seeking to save space, Multi Story Bird Cages can solve this problem. Some cage manufactures like Prevue now offer cages that are designed to be stacked like the Prevue Clean Life for smaller birds.
Remember by introducing a second (larger) bird into your household, you always run the risk of the two birds bonding and cutting you out of the deal. We hear of that especially with Amazons.
Windy City Parrot Inc.
Simply Everything for Exotic Birds