Hi,
I do following steps to target synonym keywords & keywords ranking improve for both keywords (synonym).
1>I do use both keywords in meta title {one keyword - second keyword |modifiers(e.g cheap, buy, best)}
2> I use one keyword that have more search volume in H1 tag.
3> Place both keywords in webpage content and I would suggest you to long webpage content (at least 500 words).
4>I use both keywords as anchor text for getting links and also append modifiers for anchor text variation.
What I have shared above it is my personal experience I hope it helps in your case as well but not very sure because I am not aware of keywords, webpage content , link building methods etc.
Thanks