a common join phrase for two artists collabing is “Artist A x Artist B”, and I’m wondering which character would be proper to use there. I’ve seen the letter “X”, both upper- and lowercase, or U+00D7 (multiplication “×”).
there’s also many more X marks on Wikipedia. if we’re basing it on looks alone, I prefer U+00D7 or maybe U+1F7AA (🞪), but that may be using those characters in the wrong way.
On Japanese releases, they usually use MULTIPLICATION SIGN, which is usually read as CROSS (said in English): Artist A × Artist B (Artist A cross Artist B).
I think even outside Japan, the intent is MULTIPLICATION SIGN (×), signifying the collaboration is much more than just an ADDITION (+) PLUS SIGN. 
I don’t think it’s that definitive, there are cases where it’s definitely a letter X being used on a cover. For cases like the Japanese cover linked above it definitely makes sense to match it, but I wouldn’t generalize this to all situations.
Most often when I saw non-Japanese release, they couldn’t make it a type and print a multiplication sign, but IMO, they really meant multiplication:
Artist 1 cross Artist 2
And not:
Artist 1 ex Artist 2
I seems that English Wikipedia explains it quite like I did, in their X: Other uses chapter (excerpts):
It is also sometimes used as a typographic approximation for the multiplication sign
And:
In art or fashion, the use of X indicates a collaboration by two or more artists, e.g. Aaron Koblin x Takashi Kawashima. This application, which originated in Japan, now extends to other kinds of collaboration outside the art world. [12] This usage mimics the use of a similar mark in denoting botanical hybrids, for which scientifically the multiplication × is used, but informally a lowercase “x” is also used.
I think that’s mostly a weakness of the standard English keyboard input. On Windows in particular it’s extremely difficult to enter Unicode punctuation by typing.
In Japanese it’s much easier, as I can type かける into the hiragana input and × will show up as one of the options.