Your first step should be to contact the panhellenic advisor @ UTA to find out who is participating in COB, and get a feel for how it is usually handled. It varies depending on the school. COB events don't always coincide amongst the different organizations. You may have XYZ doing events all semester long, and ABC does them one night in January, and that's it. It's also possible that not all organizations will be participating in COB, in which case, your choice will be limited to the groups who are below total. Definitely check with the panhellenic folks to get the scoop.
If/When you do go to COB events...If you are lucky enough to receive bids from two houses, and the first to bid you is not your first choice, you can ask them to give you time to decide (this is risky b/c they may sense you don't want them, and this could lead to some resentment if you do wind up accepting their bid b/c the other group doesn't bid you). They may tell you they need an answer right away, in which case you have to decide to risk pledging a group you aren't 100% sure you want, or ending up with nothing if your first choice group doesn't extend a bid.
Good luck to you - there are alot of factors to consider (more than just the above). COB doesn't guarantee you a bid (you probably know that), and it's a tough position to be in if you are interested in more than one group during COB, but it can (and has been) done by folks on GC (BellaBerlee was faced witha similar situation, and it worked out great for her in the end--
check out her thread in the recruitment stories forum to see how she handled it)--bottom line, whether or not you should wait for a second house to bid you depends entirely on the situation...it might be a better idea to wait until the Fall and go through formal so you can see all of the organizations, then you can rank your choices after pref night and not have to worry about the risk of taking a bid you don't want during COB...something to think about. I hope you find your home!