I think you should shoot for a one page cover letter, but it's not a requirement. The goal of the cover letter is to convince the editor that the submission is suited for their journal, and send it out to reviewers, so your first priority is to make the editor read the letter with a positive mind. This is usually easier with a shorter letter, but as I said, it's not an absolute requirement.
If your letter summarizes the results, discusses the potential readership, and also explicitly addresses e.g. significance criteria, it may be harder to fit it on one page. If fluff content like author names, affiliations*, etc. spill over on the next page, I wouldn't worry about it. If you fill the next page with more text, I'd start trying to cut down on it, if at all possible. What you shouldn't do is to play with margins and font sizes. That's very transparent, and doesn't improve e.g. word count and the reader's willingness to put in the time.
*I've never put affiliations in cover letters, as it is in the paper and submission systems anyway. Maybe this varies from field to field, and between single and double blind journals.