You interview at two companies. Your first choice rejects you. Your second choice says yes immediately, then gives you time to decide.

If the first company rejects you and tells you so, you can immediately accept the second choice. If the first company rejects you and ghosts you, you instead wait a month hoping the first company will get back to you, then accept the second choice.

If you are willing to completely burn bridges, you could accept the second choice, with the intention of immediately leaving should you hear back from the first choice. While possible, I'd have some qualms about doing so.

It's very simple. Contact the first choice, ABC, and tell them "Looks like things may be moving forward with XYZ. I have a preference for ABC, but we have fallen out of touch. If I don't hear from ABC in the next day or two, I will likely be signing with XYZ."

Be specific with the details, to make it clear there is no bluff. (If they are so inclined, they could call XYZ and confirm everything you said.) And if they don't respond, then go through with XYZ.

Anyway, unless the first versus second choice thing is about something concrete like compensation/benefits, or work location, a lot of it is just in your head; you have not actually worked at either place.

Exactly this.