Seems strange that two printers would have the same problem at the same time. . There's a few Google troubleshooting topics that address this type of problem so search for HP 6400 printing Blank pages (or similar printers) having similar blank printing.

Have you changed anything recently such as installing new cartridges in both machines?

Jim.

Edit: https://h30434.www3.hp.com/t5/forums/searchpage/tab/message?advanced=false&allow_punctuation=false&q=6400%20envy%20prints%20blank%20pages