My wife and I will be traveling from Williams, AZ to see Zion and Bryce Canyon on our way to Yellowstone in early October. We plan on one day in Zion and one day in Bryce Canyon.
Thinking of staying 2 nights somewhere in between, or one night close to Zion and one night close to Bryce Canyon. We are staying in a seduced AirBbB in Williams and would like to stay somewhere where you could see all the stars at night.
We would appreciate any suggestions.

Both national parks are designated as International Dark Sky Places and so you shouldn't have any trouble seeing the Milky Way.

We are staying in a seduced AirBbB in Williams

Interesting typo.

Have you considered Escalante?

https://www.visitutah.com/places-to-...owns/escalante

I always blame typos on spellcheck-I did mean secluded AirBnB.

Yeah, I figured as much.

I would not try to stay between the 2 parks - that's the worst of both worlds. Stay in Springdale to visit Zion, and a night at Bryce - right outside the park at Ruby's or Best Western (maybe they are the same thing)....

My go to place at Zion is Cable Mountain Lodge due to immediate proximity right next to the pedestrian entrance to Zion. You park at your room and just walk into the park to get the valley shuttle. That time of year it won't be as crowded as it is in summer, so if their price is too high there are numerous other options in Springdale, and the walk in location is less important if you're packing up your car for a drive to Bryce anyway.

Depending on actual timing, you could consider something like this.
Day 1 - arrive Zion afternoon - take shuttle to the end, maybe walk the 1/2 mi paved trail to the start of the narrows. or make one or 2 other stops along the way. Save longer sites for the next day.
Day 2 - am, check out, pack up your car, and spend the day in Zion. Mid /late afternoon, but before dusk, drive the 90min+ drive to Bryce. Whole lotta open space in between. Be sure to catch nighttime star gazing at Bryce.
Day 3 - explore Bryce, and head on your way to your departure city....
Day 4 - flight - or if that's day 3, shift things accordingly....

I like staying in the parks if there is lodging available.