You'll probably be able to use the low limit screw on the FD to 'lock-out' the smallest ring if you desire. Set it up as usual, shift to the middle ring and tighten the low screw. Even if it not long enough to totally keep the cage in the middle ring position, it will probably prevent the shift to the small ring if you try to shift to the low position. You'll may get chain rub, but that will let you know that your shifter is in the low position. When you know you will need the granny, just loosen the screw like you're setting up the FD again.
I wouldn't bother with the 2 chains. Size for the big/big and you're fine. Try it out and see. Since you understand the issues and what to avoid, it won't be a problem.