I have a Canon ipf8100 44" printer and it's loaded with 36" roll paper. I wanted to print two different 16x20s, side by side so the total width printed would be 32". I created a print template within LR (v2.3) with 1 row and 2 columns, a cell size of 20x16, margins and call spacing all set to 0. When I look at the Page Setup it does show a custom size and actual size of 32x20 and I have borderless and enlarged/reduced unchecked.
What's happening is the photos are printing out left justified with a 1/4" border around the whole page. The photo on the left is printing exactly at 16x20 but the photo on the right (and it's butting up right next to the other photo) is printing at 15-5/8x20.