To answer your question about the uv:you can buy canopies for the ex tera and put in either a 2% or 5% uv strip. These will not harm your crestie, but if you go for a 5% only have it on a few hours a day.
As for introducing them/getting together. First off, never buy 2 babies together. This is because you can not guarantee their sex and if you end up with 2 males then you'll need to separate them. Even 2 females can fight. If you're housing 2 make sure you have plenty of hiding spaces and room for them both and don't get a male x female unless you plan on breeding them. Introducing them later on is an option, but there are always risks. They may not get along.