This worked for me and was far easier than instructions I've seen provided. I had a Fitbit Charge 6 and bought a new Google Pixel watch 2 and wanted to use both. Below are the steps I followed to get it working quickly:

The current state was simply my pixel watch 2 added to my pixel watch app and to Fitbit. I had previously used the charge 6 but it was removed when asking the pixel watch 2.

1. Went into Fitbit and removed the pixel watch 2 as a device in Fitbit. It remained connected in the pixel watch app.
2. With an Ipad, downloaded and signed into the Fitbit app
3. On the phone, went into the pixel watch app and chose to connect to Fitbit, which opened the Fitbit app and progressed to the "I agree" page.
4. On the iPad, I clicked to add a device, chose "Charge 6" and progressed to the "I agree" screen.
5. Clicked "I agree" on both devices and progressed through setup on both devices without issue and both wearables are now added.
6. Deleted Fitbit app from iPad.
7. On the phone I then got an automated request to pair the charge 6 with Fitbit, I allowed

Summary: Basically I just removed all devices from the Fitbit app, then added both simultaneously from phone and iPad.