include multiple files in YAML `in_header:`

In an .rmd document that used to compile cleanly with the YAML lines,

output:
  pdf_document:
    fig_caption: yes
    keep_tex: yes
    number_sections: yes
    includes:
        in_header: mystyles.tex spacing.tex
 ...

I now get a pandoc error like mystyles.tex spacing.tex cannot be found I'm now using R 3.3.2 and R Studio 1.0.153. I had also tried several other forms (that gave different errors):

        in_header: "mystyles.tex" "spacing.tex"

        in_header: mystyles.tex
        in_header: spacing.tex

It works if I merge to LateX code in the two files, and just use

        in_header: mystyles.tex

What is the syntax for multiple files in an in_header line? Has it changed?