Hi,
Thank you for your interest in Essential PDF.
I am not sure if I understand your requirement right. However if you are trying to save a pdf document to stream in Flow model [PDFLogicDocument], create another pdf document using Grid model [PDFDocument] and then merge this two files from stream in the same aspx file then it is possible with Essential PDF. Here is sample which does the above:
MergeDocuments.zip
Please let me know if you have any other questions.
Thanks,
Ajish.