I use a rather unconventional method of cutting multi-opening double mats.
I size the boards as usual, then I attach the undermat to the top mat with just enough ATG to hold them together and lay out the measurements for all the mat openings on the back of the undermat. I cut these openings first. When you lay out the mat openings, make sure that you register these measurements to the edge of the top mat an not the undermat.
After the undermat is cut separate the two mats. You will see cut lines on the back surface of the top mat at every location where you cut a mat opening in the undermat. If you want a 1/4" reveal, simply measure out from those cut lines 1/4" all around each one of the cut lines that define the openings. You need to take enough time here to make sure that each one of the lines you draw around the outside of each opening mark is precisely 1/4" away to the outside of each cut mark on each mat opening position. You can adjust this measurement for any other size reveal that you desire (1/8", 3/16", etc.).
After marking all of the mat openings as described, you simply cut all these openings using the lines as guides as you did on the undermat. When you are finished place this mat over the undermat and line up the mat openings. They should all be precisely as close and even as the lines you had drawn on the back of the top mat.
Attach the two mats together with whatever method you normally use to attach mats and you should have a double mat that has even reveals around each opening like you have dreamed of for the past couple of how many (insert appropriate length of time here.)!!
Give it a try and good luck.
Framerguy