Redirect 301 running domain to another running domain [SEO]

I let’s say I have landing page a.com and b.com (similar content & brand but different topic), these 2 are running and ranked on SERP, got traffic, but b.com is better than a.com in serp with specific competitive keywords (it can be on page 2-3).

The question is, when I redirect 301 b.com to a.com, will a.com replace the b.com in serp and get better ranking? (maybe jump to page 1), or something else?

Thank you.