There's a lot of good ways to do it already mentioned. But what I do is this...
- set it to print centered on the sheet (on the printer not the RIP)
- set it to measure every load, using the "minimal" setting. Minimal will only measure the leading width and find the starting edge, which is what you want in this case.
- print the first side and eject to input side
- flip the sheet over in such a way that you are using the same leading edge of the coroplast....this step is the key. Because it's centering based on the measurement it took from the leading edge, and since you are using the same leading edge it will center in the same location for the back side (I dare say almost perfectly in my experience)
- print the back side, keeping in mind you will need a second print file exactly the same as the first but with the signs rotated 180 degrees.
I actually developed this method for printing 2 sided PVC with a large amount of small units (full bleed with borders in the design) ganged up on a 4' x 8' that will then be cut on a CNC router but later found it great for yard signs. The reason it works is because it's centering to the same leading edge on both sides of the coroplast and not measuring for skew. This way the left/right/trailing edge can be skewed all they want, and the sheet can be of varying widths/lengths from sheet to sheet (typically they are closer to 97", sometimes over 48" and never square or consistent) because all loading/centering is done based on the same leading edge. The biggest disadvantage to this method is it requires and extra cut during trimming because it's now leaving roughly 3" on the left/right instead of 6" on one end. But it will line them up nicely and if that's what it takes for a successful print then one extra trim is no big deal. I like this method because it's basically fail safe once it's set up properly. I've tried buying "square" sheets for an additional cost and they are more square but not square enough to accurately line up bordered, full bleed prints from front to back on a full sheet . Hope it helps!