I wish I had the option, too.
BUT I don't think it's a Solus thing. In Solus all I have is the Brother driver installed. Not enough.
In W7 when I installed the Brother Bloatware (that comes w/printer) I did have 2-sided printing.
In W7, for another printer, I refused the bloatware install, and did not have 2-sided printing.
In other words, this advanced option was there and then not there.
But we all remember things the way we remember them. I'm often wrong.
Trick from the old days, of course, is for a 10 page doc, print pages 1 and 3 and 5 and 7 and 9.
Flip stack in tray. Then print the even numbers for perfect 2-sided printing.