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I have one baby RES in a 29 gallon tank right now. I was thinking of getting another turtle soon (maybe a yellow bellied one)

Would they get along?
Would they both fit in the tank when they get older?
Are there any other complecations that I don't know about?

One adult RES needs a tank between 90-120 gallons. If you add any other turtle into the picture then the tank size will have to be larger. You have to remember that as your turtle grows so must his home. Minimum recommended tank size is 10 gallons per inch of turtle.

RES are not social creatures and do not need "friends" in the way we humans think. In the wild they come together only to mate or compete for the best basking spot. More than one turtle in the tank will mean more cost on your part in the long run. And you always have to keep in mind that as they grow they may not always get along and will have to be separated into their own set-up. That means two of everything- filters, heat bulb, UVB bulb, basking areas, heaters, tanks etc. Some turtles simply will not be compatible together, even if they got along together when younger.

Let me just say that if I had to do it over again, I'd only get 1. I bought 2 hatchlings a year and a half ago. They are a handful! I now have 4 in two separate tanks. I'm very lucky that they haven't shown any major agression issues because then I'd need at least 2 more tanks. That's a LOT of work! My advice is to concentrate on taking really good care of the one you have