As per the description above, you are not able to combine/merge one Certified PDF file with another PDF file, Is that correct?
Sadly, a Certified PDF file cannot be merged/combined with another PDF file.
A Certified Document provides PDF document and forms recipients with added assurances of its authenticity and integrity. The entire point of Certification is that a certified document can't be edited. If you want to merge it with another document (unless you use a portfolio) you will first have to remove the certification.
For more information about combining/merging PDF files, you can visit: https://acrobat.adobe.com/in/en/acrobat/how-to/merge-combine-pdf-files-online.html
Feel free to update this discussion in case you need further assistance.