Is there a way to get the inbuilt print command to print multiple files all thru the same Windows' Print Requester?
It's up to whatever is the Print handler for the type(s) of files selected. All Opus seems to be doing is passing the files to the print handler.
Some handlers work with multiple files; I guess some others don't.
e.g. If I select three BMP files in Opus and click the Print button, I get the Windows "Print Pictures" tool pop-up, and it offers to print the images one per page or several on the same page. That's before the printer or printer drivers themselves have gotten involved.