That's an interesting point, and one I've not thought of. However, I wonder if you looked for them, if you could come up with just as many important moments when they aren't filmed that way?
I admit I'm shooting from the hip here without having read saucy's post, but the first thing that popped into my head is that aren't they both in profile when Ennis tells Jack that all those things Ennis doesn't know might get Jack killed if Ennis comes to know them? Or, maybe that shot sticks with me because it's filmed differently? And we see them both full front when Jack comes out of the tent and Ennis is getting ready to go back to the sheep after TS1.