Thanks @Janno,
Update: when I posted, I think I had misunderstood that your query over multiple content types worked, hence the possible conflict. Now I think your query does not work; there is no conflict and I agree with you that such queries should be possible. The remainder of this is my original post.
When I researched this issue, I found something that seems to conflict with what you wrote:
Gabriel: "However, the issue here is that each field is defined on the basis of a single content type.
"That being the case, although your fields may have the same name across multiple content types, they will effectively be different entities, so it’s unfortunately not possible to search through the fields of different content types that you may have specified in your query.
“So, as Charlie greatly explained, you may want to create a common content type.”
Is Gabriel incorrect here, using a different API than you use, or something else? @Janno, maybe you could comment on that page?
It seems that Contentful could release some documentation around best practices here, as this is an extremely common case (for example, find content entry by URL, for which there may be some other solution - I am not fluent in Contentful).
Thanks & regards,
-John