Kids switching between two worlds

Question

There's a story that I read in junior high in the early eighties, that I'd love to find again. I remember a bunch of scattered things about it, the way one does with dreams. So, this post will be likewise scattered.

• It was a YA title, with junior high protagonists.
• An old man gives them an – amulet? – that when held and turned sideways (like a key) sends you into a parallel world.
• That world has a 1-to-1 spatial relationship with our world; at one point the protagonists get past some obstacles in the other world by bicycling past them in this one.
• They need to get the amulet to a woman; I think her name was Stella. The villain is responsible for most of the obstacles in the other world blocking their way. Except the final really long set of stairs. The villain claims that Stella(?) was responsible for the stairs.
• They are attacked more than once by bipedal humanoid creatures with horse-skulls for heads. Later, when the protagonist claims to have killed one with his pocket knife during their escape (which I'm vaguely remembering to be aerial, somehow), he is told "Fool, you can't kill death!"
• There's a scene where he takes some of the kids who make fun of him at school to where he knows the horse-head things will be, betting them that he can show them something that will scare them off. Indeed, they flee in terror, but the next day at school, they claim that there was nothing there, and it was the protagonist who fled, and they are mocking him for it (like they were really going to admit to the other kids what really happened?).
• At the end, when the protagonist climbs the final stairs and reaches Stella(?), he hesitates before giving her the amulet, unsure if this is the right thing to do, or another trick. She sees his hesitation and smiles, waiting patiently, "content that he should choose". And it was this that convinced him that she was the "good guy", and so he gave her the amulet.

There's a chance that I'm mixing a couple of different stories together, but I think that it was all the same book. And as far as I was aware, it was a stand-alone, not a part of a series.

I have no idea what the title was or who wrote it, so there's no way for me to search for it by conventional means.

Answer 1

A chunk of that sounds like Alan Mendelsohn, the Boy From Mars by Daniel Pinkwater. It's a YA novel with junior high protagonists, an old man (Samuel Klugarsh) who doesn't give them an amulet, but does give them the means to enter a "parallel plane of existence". That world has a 1-to-1 spatial relationship with our world; at one point the protagonists get past some obstacles in the other world by bicycling past them in this one. At one point, amusingly, one protagonist gets "trapped in both worlds at once" and finds it very difficult to move around, as he has to avoid obstacles in both worlds simultaneously.

However, nothing else matches the description, as far as I can tell.

So it's possible you're conflating two YA novels, or it's possible I'm guessing wrong and there is another book that has those same elements as Alan Mendelsohn but also horse-headed monsters.