Header Merge

You cannot merge them. Hard-coded into the application. You can click on the Document word in the Pages v5 toolbar, and switch of the header entirely.

Then you insert a one-row table with one or two cells scaled to stretch across the previous header location. At this point, while you have a one or two segment header replacement, you might want to save the document as a template (e.g. Header 1-cell, etc.), so you have a reusable solution.

For the current document, if you want this custom header to appear on all section pages, then you must select your custom header, and then:

Arrange menu : Section Masters : Move Objects to Section Master.

This will place your custom header in the background.