12.Changed his stage name to an unpronounceable symbol
Michael Jackson
✓
Prince
✓
The symbol, known as the "love symbol," was adopted in rebellion against his record label, Warner Bros., after they trademarked his name
13.Has the best-selling album of all time
Michael Jackson
✓
Prince
✓
The album in question is Thriller, which has sold 70 million copies.
14.Played guitar
Michael Jackson
✓
Prince
✓
Both
✓
15.Was in a band with his brothers
Michael Jackson
✓
Prince
✓
Both
✓
The Jackson 5 released hits like "ABC" and "I Want You Back" and is how Jackson got his musical career started.
16.Wrote "We Are the World"
Michael Jackson
✓
Prince
✓
Both
✓
Interestingly, there was a part of the song that was meant to be a duet between Jackson and Prince; Prince refused, possibly because he didn't want to sing alongside other famous acts.
Maybe they aren't scientifically, but in common usage they are. Saying something like "Had a pet ape" doesn't have the same ring to it. At any rate, I think it's obvious what the question is asking for.