The rendering in ViewerJS is done by WebODF for ODF and PDF.js for PDF. If you would like to help with the ODF rendering you can joinwant to ask how I can make adjustments to the layout so that it displays
two documents side by side.
https://open.nlnet.nl/mailman/listinfo/webodf
If you want to display two documents side by side, you can add two ViewerJS instances to your page.
Best regards,
Jos van den Oever