I have a sewing pet peeve. It is peter pan collars that don't meet at the center front neckline. I have seen them on garments in magazines occasionally and I find it hard to think that look is intentional. Perhaps if there is a design detail like a button or bow, you may want to separate the collars. But, IMHO, if it is a basic Peter Pan collar, it should match. Now, truth be told, I made this mistake myself for quite some time when my children were small. Then the light went on. YOU JOIN THE COLLARS AT THE SEAM LINE BEFORE SEWING. What I would do back in the day was meet the CF edges of the collars at the fabric raw edge, not the seam line. Then of course, when my machine went over it, the collars would be spread a good half inch apart at the seam line. Maybe I should have been brighter from the get go but in those days it was make it fast and make it many. I don't sew like that any more. (30 yrs ago)
First, I find the exact 5/8 inch spot on the collar CF edge. I mark that with a pin or whatever marker you like. Then I tape the two collars together with their 5/8 inch markings kissing. You can see the tape shining. Next I go to the machine and do 3 or 4 wide zigzag stitches on the markings . This is to stitch the two collars together. Pull off the tape. You will now be able to pivot the collars so the seam allowances overlap properly.
There is a lot of free play here in the SAs. Now do your normal pin and stitch the collar taking care to place the zigzags a hair below the 5/8 SA at CF.
Remove the zigzags, press, and voila! IRL, you really don't see the bottom collar peeking out, a camera angle thing!....Bunny