I think butt cheeks look better when spread apart.

On a meta note, I think this is more of an uncle joke

Linguistically speaking, there are three tests that can help us determine whether "buttcheeks" is one word:

1. The lexical drift test. Note that a "blackboard" doesn't have to be black - the ones I grew up with were mostly green. Once two words have univerbated that word is then free to drift in meaning from the meaning of its original constituents.

In this case the lexical drift test is inconclusive - a "buttcheek" is still the cheek of your butt.

2. The initial stress test. In English specifically, compounding tends to result in a stress shift: "a black BOARD", but "a BLACKboard". There are exceptions to this tendency, though, and so it's not the best metric to rely on.

Even so, we can note that "BUTTcheek" does pass the initial stress test.

3. Finally, the best test is the modification test. Only the head of a compound can be modified - "pink toilet paper", for example, can only refer to pink paper, and cannot mean "paper for a pink toilet", which lets us know that "toilet paper" is really one word in English, not two.

"A green buttcheek" does specifically mean "A cheek that is green", and you could potentially have a butt where only the cheeks are green, but not the entire butt, so "buttcheek" does pass the third test as well.

In conclusion, "buttcheek" is one word in English. Note that this only describes how this word exists in speaker's minds, and should not be taken as advice on whether to write "buttcheek" as one word or two.

Writing is not language, and writing conventions are arbitrary and rarely follow the actual rules of a language, and so linguistics has little to say about whether a prescriptive English teacher would recommend spelling "buttcheek" as one word, with a space, or with a hyphen. But regardless of how you choose to write it, it is demonstrably one word in English for the above reasons.

It's a compound word, you should slam them together.

Needs a dongle between.

For authenticity you could leave a hole in the middle.

JFC, dad! I'm calling CPS.