Why do you need two png files in one? png's are pictures. Best bet is to create an rtf file(ods rtf file=, before and close after) then both images will be in a useful file. Png has no concept of pages and such like, so your output png will never be a useful file.
Then your stuck with a very silly process and a client who does not know what they are talking about. Just increase the cost of the task and manually open the graph in a paint application, extend the size of the image, and copy paste the other in there. A PNG is a picture it is not a document, it has not page operations, or document metadata, therefore is not useful outside a proper document format.
You can use SG annotation in proc sgplot or DRAW statements in GTL to insert graphs into graphs. The link above shows one type of example.
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