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I have been wondering about tank size for the future. I know that its 10 gal per inch of shell, but is that per turtle? If they both grow to full size am I really going to need a 240 GAL TANK!!! I am planning on getting a 150 gal tank as soon as I can afford one but will that be too small for two RES?

Yes, that's per turtle. How big are your turts now? You don't need to go that big right away unless you really want to save money later. I have my two 6in res in a 125 right now and that'll do them for a while.

150 gallons are fine for two turtles as long as both are under 7 inches. But for two full grown turtles you may need more room than this depending upon whether you have 2 females that reach 10-12 inches each. Males reach between 7-9 inches but are more likely not to get along together in the same habitat and will have to be separated if so.

Ideally that's what would be good for a turtle (and we all want the ideal for our pets, don't we?). But I've read that for more than one turtle, it is possible to allow half as much space for the second one (example if you have a 3-inch turtle in a 30 gallon, adding a second turtle of approximately the same size would require at minimum, a 45-gallon tank). It can be quite easy to abuse this guideline, though. For multiple turtles, I would go as big as possible.