I imagine that comes with its own legal issues (beside coming up with an ultimately bogus license agreement between two entities that are both not wanting to cede the trademark), and one company using it “with permission” (and essentially giving up the trademark themselves), yet using it without referring to the other’s comics/characters, would reveal the trademark to be the farce that it was*. But yeah, that they didn’t even bother with that is another point indicating they both knew the trademark claim didn’t hold a lot of water, and they were relying purely on the threat of legal action to maintain it.
*My understanding of trademark isn’t terribly deep or nuanced, but surely if Marvel, say, was the ostensible holder of the trademark, and DC use it to refer purely to their own comics and characters, then it’s diluting the trademark, even if used with permission. Both parties would be essentially admitting that “superhero” wasn’t something unique to, and identifying of, Marvel products. Seems like it fundamentally becomes the same problem as claiming a co-trademark. “It’s a unique identifier for us and us alone.” “But also us.”
