Two cages are best, especially if you don't want to breed the dragons at all. Housing a male and female together all the time, even if both get along, means lots of eggs, as many as 100 a year from a single female. (100 a breeding season is probably average, but you can have much more or less as well). Producing eggs and having a male around wanting to mate as often as he can, puts alot of stress on a female, causing them to loose weight, develop parasites, loose calcium and with proper care isn't given, eventually it will kill her at worse or drastically reduce her lifespan at best. It is much easier to build to stacking cages than have to put in the extra care and money to deal with a constantly pregnant female. (to keep her healthy, extra insects and calcium/multivitamins, etc). Not to mention the work and money needed to incubate and raise the babies, finding homes for them etc.
I also suggest the height of the cage be at least 16" with 18"-24" being the better range. Depending what you plan to use as your basking spot and UVB lights, 16" is fine. But the basking areas will need to be fairly shadow, to ensure the dragon doesn't have a chance to jump at the basking bulb or UVB bulb and possibly knocking it down, breaking the bulb or burning himself.
12" height is just too low...you need 6-8" between the dragon and the bulb to ensure the dragon can't get into contact with it. ALso you want to provide some climbing areas for your dragon and 4" is probably the most you could provide in height, in a 12" cage (allowing a couple inches for the thickness of the UVB and basking fixture, if both are inside the cage, and not sitting on top. Having stacked cages you almost do have to have the fixtures inside the cage, or the top cages will need to have 'feet' so there is enough room to get at your fixtures for the bottom cage. If this has to be done, easier to just add the extra 4"-12" of height for the cage so you can mount fixtures inside)
-----
PHLdyPayne