I noticed that if using the Nutaku version of Harem Heroes, you never get cross-promo banners. Or any ad banners at all, for that matter, which is usually a good thing. But in this case, not so much. To test it out, I opened Hentai Heroes in a separate tab and was able to see the cross-promo banner. Unfortunately, that account is not my primary and I haven't touched it in years. At any rate, that helped me confirm that I don't have an ad-blocker running.
So, knowing that I open both games through the Nutaku website, they clearly are using the same email address. Not really an issue since I only have the one email address anyway. My Hentai Clicker has gone beyond stage 3000. There is no ad-blocker functioning here. And yet I NEVER get the cross-promo banner. So there is apparently NO way for me to have Mizuki join my harem in Harem Heroes.
They need to get this fixed, or figure out a different way to implement cross-game functionality.