Well, I've always housed mine together

But I'll get into that later.
To answer your question:
-You would need a female, as males will fight each other.
-You need to find one of similiar size as yours. ( as someone else mentioned )
-You need to be prepared in case the dragon does not get along with yours, which is a possibility since he has had the reign for 1.5yrs now, he may not like having someone else thrown in with him.
-You should have the new one in it's own tank for quarentine purposes, then once your sure the health of it is fine, then move it over.
- If you get a male/female housed together, they will breed. So you should educated yourself on that matter. Especially on how taxing eggs can be on a female dragon.
As for my situation: I have had dragons for 6yrs now and I have bred them.. I house 1 male and 3 females together. 2 of the females went in together as juvies....( under 6mos old ). When they reached breeding size, a male was introduced. Then the 3rd female came. I watched them carefully for any signs of aggression because they were all adults now.
Some people will flame me for this one, but the 4 are kept in a 4ft x 2ft x 2ft tank. ( some will say that is way to small for 4 fullgrown dragons ) They all have their own basking spot, as well as they will bask together ( share the same branch, but there's adequate basking area's for 4 of them ).
Onto the only negative experience. The first 3rd female developed something wrong in her brain ( a tumor perhaps... I did not have a necroscopy done so I can't say what exactly).. she started acting very aggressive towards the others, nipping and whatnot. So I had to remove her to her own tank. One day I was cleaning tanks and I put her in with the others for a moment as she had been fine with them in the bath. I turned, and looked back and she had chomped on one of my other dragons tails, crushed it. I had to pry her mouth open to get her off. I put her back in her tank where she proceeded to literally eat her own leg off. We had her euthanized. ( The dragon she chomped on, had to have 2" of her tail amputated )
I relate that experience so you know that "things" can happen.... ( this was a physical illness that caused my dragon to "lose it"... but anything can happen and you need to be prepared in case ).
Personally, I see nothing wrong with housing multiple dragons, as long as you are prepared to deal with the "what-if's".
Just my opinion though.