I am trying to print on both sides in macOS without success.
I make sure to check "Two-Sided" and also made sure to enable long edge on the printer control panel.
What else can I try?
(attempting to print from tray 1 using 12" x 18" cardstock.)
There hasn't been a driver since Mac 10.10, what are you runing and what driver are you trying to use with it?
I just tried this one for 10.10 and it appears to work on 10.14.1