KEYWORDS: Can I use synonym keyword from one page as a focus keyword for another page?

Idealy not, you’ve chosen the phrase as a synonym because it’s not as good at the focus keyword.
It’s not in a ‘gang’ with that focus keyword, for that particular page, post or product.
It ideally shouldn’t be used again for another page or it will confuse Google.

BUT of course this depends on the length. If the synonym is what you do. Financial copywriter for example, you’re going to use that phrase again and again. Keyword double up is really only an issue for longer tail keywords, like financial services copywriter for insurance websites. You wouldn’t want to use a phrase like that across multiple pages.