Nine crimes you have no right to tell people what to believe, practice, and/or study. If astrology is not real to you, I respect that as do others. But you should respect that others do hold astrology as valid.
I think you're talking about Decanates. A gemini born from May 21-May 31 is a "gemini-gemini". A gemini born from June 1-June 10 is a gemini-libra. Then, all other geminis would be gemini-aquarius. I'm born june 19th, so according to decanates, I'm a gemini-aquarius. I think for me this seemed so true because uranus conjuncts my ascendant. I found great intrigue in the study. But for other gemini-aquarius's decanates may not seem so true because uranus may not be prominent.
In Decanates, a gemini-gemini will no doubtedly act the most "mercury-like" of the others. They love to talk and are true geminis at heart. A gemini-libra shows a more sensitive and artistic side. They love to talk but are more artistically inclined and appreciate beauty more than other gems. Gemini-aquarius is the most eccentric of the geminis. They are more inventive and original, with rebellious nature.
Basically, something easy, is to take gemini and incorporate the other sign in with it. In other words, act like a gemini-gemini would be similiar to a gemini ascendant, gemini sun. Gemini-libra would be like a libra ascendant, gemini sun and gemini-aquarius an aquarius ascendant, gemini sun. Combine the characteristics of the signs.
Hope this helps!