Generally, Google wants you to use one rich result structured markup (schema) per page. Why? Well, if you use multiple, you won't know which Google may show and you probably won't to define that on your end, versus letting Google define it for you.
John Mueller from Google was asked if Google has some sort of priority of rich results it will show. So if you you have both FAQ schema and review schema on the same page, which will Google show in the search results for that page.
John said "there isn't a defined order of priority for rich results types, and in many cases we don't show multiple types for the same page." John added "if you have a preference on a page, I'd recommend focusing just on that." So yea, focus on adding one set of schema/structured data per page and try to control what rich result Google may decide to show, if Google decides to show it.
Here are those tweets:
There isn't a defined order of priority for rich results types, and in many cases we don't show multiple types for the same page. If you have a preference on a page, I'd recommend focusing just on that.
— 🍌 John 🍌 (@JohnMu) August 30, 2021
And if it was me, I'd pick reviews or product schema over FAQ schema - one should have a higher CTR than the other. But you can test both and see what drives a better click through rate from Google using the Search Console performance reports.
Forum discussion at Twitter.
