Either kitchen/bath sharing one plumbing wall in one dwelling or separate dwellings backing up to a common plumbing wall; also identical dwellings, right and left, backing up to each other.
In our business if we sell and buy "back-to-back" we mean the material which we have purchased and the material which we have sold is intended to be the same identical material and the conditions of the contract insofar as tonnage, shipping time, grading, and all other matters, except for price and terms are the same. Back-to-back contracts, both the sale and purchase, have identical cancellation dates. Back-to-back contracts "cancel each other out" as far as position is concerned.
Two or more items that are placed in mirror image to one another. When used with plumbing fixtures, where a common wall is shared, back-to-back installation saves both space and money. Since the water supply and the waste drain and pipe venting are shared, repeat installation is unnecessary.