How to rank both main keyword and Secondary keyword on Same page

There's a fine line as far as double/triple SERPs. You have to get the backlinks in place & rank the 1st page then piggyback off the ranked page authority to rank the 2nd or 3rd page for the same keyword.

Do not use the same page titles or on-page text for anything involved or you'll end up with one page ranked & everything else buried in Supplemental SERPs for your target keyword.

Once the first page is ranked then use keyword anchor-text internal link pointing to the 2nd internal page that's targeting the same keyword.

Either way you can't skip the backlinks on the first page. You said you only have forum links which odds are they're all nofollow links that will never rank a page. Build better/followed links.

The end goal is to create at least one authority page for the keyword, from there you're simply spreading the authority to other internal pages via internal links.

Research SEO silos, that's how I get double/triple SERPs per keyword.

Also, competition plays a role in double/triple SERPs, the tougher the competition, the stronger your own webpages have to be to rank for each SERP position.

My advice is start with very weak competition keywords, figure everything out, get a few double SERP listings & then work towards tougher competition keywords with the same process. That way competition isn't a factor in the learning process.

If your keywords are both already ranking and moving around first few pages of the SERPS, then your on-page SEO is probably good. Now all you need to do is analyze your competition of first page and out do their link profiles. By the sound of it, a few good links per month should do the trick to get you to rank 1.