Combining multiple first pages is a common task in form and document filling, as well as archiving. It can be done in two snippets:

for file in *.pdf ; do pdfseparate -f 1 -l 1 "$file" "$file-1.pdf" ; done
find . -name '*-1.pdf' -print0 | xargs -0 -J{} pdfunite {} out.pdf

This combines first pages of all PDFs in the directory, and outputs to out.pdf.