I built a copy years ago, and am still using it. Does cut the raking time a fuel in half, but leads to a temptation to over use the rake.
As M Nut says, a pain to transport on the road, and heavy to switch the rakes around if you don't have front dollys on them. The small swivel wheel also goes bananas if you go down the road too fast with it.
Width can be a problem if your fields have many obstructions, or narrow gates. If you are re-raking heavy windrows, the hitch on the rakes will sometimes catch a big pile of hay and drag it.