What is the difference between an AC gain pedal and an RC gain pedal if used individually, or is a combination pedal with both better?
It looks like you are asking about specific brand names of boost pedals made by a company called Xotic California.
The RC Booster pedal is designed to increase the level of your signal without changing the signal at all. That's what is meant by "transparent" - no tonal change.
The AC Booster pedal is designed to increase the level of your signal with a tonal change. According to the web site, it gives your guitar a "warm and pleasant sound". The AC Booster can also be turned up to create distortion.
There are a few other differences between the pedals. The terms "AC" and "RC" for boost pedals are only used by Xotic, as far as I have seen. They are just the brand names for those pedals and I haven't found out what they mean or stand for.
My only guess at this time is that "RC" stands for "Really Clean" (not coloration of signal) and "AC" stands for "Almost Clean" (slight coloration of signal).