I make those very often. It's easy to see but hard to explain. Mo ci provo:).
When you clean a fresh anchovy, you stick a thumb in its bum and separate both flanks along the tail from the spinal cord – down to the tail itself. Then you gently tear out the spinal cord together with the the tail, the innards following, up to the head and take everything out together with the head.
What you get is two halfs of the fish held together by nothing more than a stretch of skin along the back. So, a good chéf places them on the plate flat open. The problem is that skin is very thin and the two halves often separate. So, an amature just plonks them onto a plate as they come, but a good chéf "le appaia", that is, puts them, as they say in Quebec, "side by each" as if they were still held together by that thin strech of skin along the back, OK? The fish lies on the plate looking neatly opened-up. So, you don't put little fishlettes (:)) in pairs but neatly couple/match their halves. Woa! Hope to have been of help.