Did you wonder if they are interchangeable when referring to repetition? If so, they are, without considering shades of meaning. Although they are similar words, each word's nuance in sentences can differ when they are used. As to 우열님 mentioned, I agree with him that 또 can modify 다시 because 또다시 has more intenser meaning than 다시.

In my best guess "또" is a modifying "다시" just like "again" and "all over again." Only the difference is, I think, you can use "또" without "다시." "또" acts alone meant the same thing. For example, "또 해봐" = "다시 해봐" Good luck

No sentence at all, I was just wondering if they were or not.^^

I think usually they are interchangeable, but I can't overgeneralize the rule. In which sentence do you think 또 and 다시 is interchangeable? Or where did you get that idea two words are interchangeable?