What are the two meanings (senses) of the word “he” in your sentence?
Dan or Steve is what I mean here — meanings within the context of their usage, not in an isolated sense. These meanings would both be described as “third person singular male pronoun” in a dictionary, but by the nature of a pronoun the whole point is for it to refer to something you’ve already talked about in context
The multiple senses of the pronoun in my sentence is the cause of the problem, not the multiple subjects.
We can test for this. If there’s only one subject, Dan, then the sentence becomes:
“I was with Dan (they/them) the other day. They hadn’t brought a poster they needed and went back to the car to get it.”
No ambiguity there, it can only mean Dan. Similarly, with a single subject that consists of multiple people:
“I was with the newlyweds the other day. They hadn’t brought a poster they needed and went back to the car to get it.”
Exact same thing, no ambiguity. So we can use “they” in both senses and it’s totally fine so long as there’s only one subject. The ambiguity comes about when there are two possibilities already mentioned that the pronoun could potentially refer to — just like if Dan and Steve are both “he”.
Dan or Steve is what I mean here — meanings within the context of their usage, not in an isolated sense. These meanings would both be described as “third person singular male pronoun” in a dictionary, but by the nature of a pronoun the whole point is for it to refer to something you’ve already talked about in context
We can test for this. If there’s only one subject, Dan, then the sentence becomes:
No ambiguity there, it can only mean Dan. Similarly, with a single subject that consists of multiple people:
Exact same thing, no ambiguity. So we can use “they” in both senses and it’s totally fine so long as there’s only one subject. The ambiguity comes about when there are two possibilities already mentioned that the pronoun could potentially refer to — just like if Dan and Steve are both “he”.