Hold on... I need to whiteboard this.

OK, now a serious response.

What you are describing, to me, is how people should vote. It's normal voting behavior. In realty, there are dozens of issues people care about to varying degrees, and you can assign values to each issue (how much it matters to you), add them up, and vote based on that.

My issue is that single-issue voters assign infinity to one issue and vote based on that, which is both usually lazy and stupid. There are cases where it's reasonable, but they're rare; if Trump supported Palestine and Biden supported the genocide (which tells you which side I'm on on that topic), then yeah; I think genocide is a reasonable single issue to make a decision. But in this case, Biden is pro-Israel, and Trump is pro-genocide (he's said he thinks Israel isn't going hard enough), so pro-Palestine voters should vote Biden.

Going back to your example: if two candidates do have the same position on issue X; and candidate A supports Y and Z; and candidate B doesn't support Y or Z, then even if your single issue is X, you don't just not vote. You have an opinion about Y and/or Z, so you vote for A or B based on that. And in your specific example, first: there is no candidate C in the US; there hasn't been since Abraham Lincoln. Voting on the US is fucked up, and a vote for a third party is a wasted vote: not a protest vote, but a wasted vote, b/c C has zero chance of winning, and you're taking your vote away from one of the other candidates, one of whom is more aligned - even if only slightly - with you values. Second, it would be unusual if you cared about X, Y, and Z equally, so one of those two candidates is going agree with you on one of those topics which is more important to you, and you should vote for them. Or - and this is the real situation in the US - two candidates are very similar about a half dozen issues, but widely differ about another dozen pretty important topics. And although that long tail of issues may not be your triggers, the weight of all those issues should make it clear which guy (and, so far, it's always a guy) you prefer.

Biden and Trump agree on Palestine, although it's clear Trump is the worse choice for Palestinians. They agree on big business. The differ about many other important topics:

• A woman's right to choose
• Supporting Ukraine in its defense against the Russian invasion
• Universal healthcare
• Environmental protection
• Worker's rights
• Nepotism in government
• The peaceful fucking transition between administrations
• Minority rights
• Whether black people are humans or not
• Whether women have the right to not be molested

and many other "lesser" topics. Saying that you aren't going to vote because Biden is only less bad about Palestine is making a decision about a single issue, and it is a problem; it's thoughtless, and lazy.

Yes, if you consider more than one issue (X, Y, Z) when deciding who or who not to vote for you are a multi issue voter, if you only consider a single issue (X) when deciding who or who not to vote for, you are a single issue voter. It's not per candidate, it's per vote.