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.
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.