I'm not sure what you mean by back to back? I was hoping to avoid
placing an air brick on the first floor. Also, the problem still
remains with the air brick in the attic; potentially there are two
channels behind it - would that mean two air bricks are required and
then a degree of guess work when fitting to ensure that they each
perforate one of the channels behind?

I would put a vent in the wall where the fireplace opening used to be,
and then put a ventilated cap on the chimney pot.

Don't faff around putting air bricks in the loft.