If the Dan in your example used he/him pronouns and so did Steve, then it is equally unclear
“I was with Dan (he/him) and Steve (he/him) the other day. He hadn’t brought a poster he needed and went back to the car to get it.”
There’s no way to know whether the “he” is Dan or Steve. The they/them pronoun isn’t the problem in your example, the structure of the sentence is.
There’s no way to know whether the “he” is Dan or Steve.
Your example sentence is always ambiguous because there is only one sense of the word “he” but two possible objects. My example sentence is always ambiguous because there are two senses of the word “they”. The two situations are completely different linguistic issues.
Your example is of a poor speaker. My example is of a poor pronoun choice.
The they/them pronoun isn’t the problem in your example, the structure of the sentence is.
I disagree entirely.
Dan (or Steve, or both) is the subject of this sentence, not the object.
In both sentences, the pronoun used has two possible meanings in that context. That the two “they” definitions might be listed separately in a dictionary does not seem very important. It wouldn’t even need to be separate, as “third person pronoun, indeterminate number and gender” would accurately cover both cases.
What would be a non-ambiguous version of the sentence, in your opinion?
Dan (or Steve, or both) is the subject of this sentence, not the object.
You may be right about that (I’m not sure) but it doesn’t effect the argument.
In both sentences, the pronoun used has two possible meanings in that context.
What are the two meanings (senses) of the word “he” in your sentence? It only seems to have one meaning from what I can tell.
As I understand it, in both sentences there are two subjects (using your terminology) but in my sentence, the pronoun has more than one sense whereas in your sentence the pronoun has only one sense. The multiple senses of the pronoun in my sentence is the cause of the problem, not the multiple subjects.
In my sentence it’s also possible that there is the same ambiguity of subjects as in yours but that is not a given because it depends on which of the senses of the pronoun is intended. And that isn’t clear. Which is the problem.