I also have an HP Officejet Pro 6978 that I use for B&W printing only, since it quit doing color.
I did have the HP assistant program that you mentioned, but it disappeared when I installed the HP OMEN PC.
When I have both printers on, the Epson goes crazy with the printing. The same 2 lines one after another.
I now have only one printer on at a time.
Jim Musgrove