* Time period: It refers to a period of two months. For example, "Futatsuki no nagasa" (ふたつきの長さ) means "the length of two months".
* Age: In a more informal context, it can refer to the age of someone who is two months old.
* Double moon: In poetry or artistic contexts, "futatsuki" can be used metaphorically to refer to the image of a double moon, perhaps a crescent moon with a full moon beside it.
The actual meaning of "futatsuki" depends on the context it is used in.
Let me know if you have more context for the word and I can give you a more specific answer.