What a good idea. I love smart ways of doing things.
I did see a similar idea for sewing strips together using a length of card cut to the width of the finished measurement.
Say you are sewing 2.5" strips, sew the first pair together, then press open. With third strip placed R/S together put the strips under the needle with the new strip on the bottom, leaving the first seam uppermost. Now you place the card (2"wide) with the left edge butting against the first seam and you follow the R/H edge with your new stitching. Voila, you have sewn the second seam exactly 2" from the first.
Try a test piece first. You may need your card a tad wider if your thread and/or fabric is thicker and you 'lose' a little in the opening out of the seam, IYSWIM.