How to add a new page to an existing HTML document with R flexdashboard

Question

I have a large R markdown script (first.Rmd) that generates an HTML file using flexdashboard

---
output: flexdashboard::flex_dashboard 
orientation: columns 
vertical_layout: scroll
---

I would like to add another page to this HTML but using another Rmd script (Second.Rmd) executed at a later stage.

If in the YAML of Second.Rmd I put

---
output: 
  flexdashboard::flex_dashboard:
    includes: 
      before_body: output.html
---

I just get a small blank square. If I write

---
output: 
  html_document:
    includes:
      before_body: output.html
---

it works better, the page appears where intended but it's content is shown at the bottom of every existing pages.

How to add another page the an existing HTML document?

Thanks!