I’m trying to emulate a shorthand for 2 x six notes notated as two dotted half notes in a 4/4 bar with a 6 under each note and a strike over the stem. Can you suggest how to this fairly common notation?
Start a 6:4e tuplet, enter the two dotted halves. Then select the measures and do Shift+R, /, Enter.

The only catch is that Dorico does not center the number over the stem, which I think is generally how you see this notation; you’ll need to move the numbers yourself in Engrave mode.
I enclose my effort. When I started the tuplet I had a quater note selected.
Test_repeat.dorico (1.3 MB)
Note the e in @asherber 's post:
The e stands for eighth note, which will give you an eighth note sixtuplet no matter what note value you have selected. If you enter just 6:4, you’ll get a sixtuplet for your selected note value (in your case a quarter note). Alternatively, you could first select the eighth note value before entering the tuplet.
I always keep this file at hand. On pages 37 and 38 you’ll find some info on the Tuplets popover and how to use beat units in popovers: