Putting two larvae in each cell is not the same as double grafting. Unless I am misunderstanding, putting two larvae implies putting them both in at the same time and letting the nurse bees select which larva is raised to completion.
Double grafting involves starting a queen cell with one larva, letting the bees start to raise it and after a few days removing the started larva and replace it with another one, In theory, this gives the second larva its start at development on a thick bed of royal jelly, with no waiting time till it feeds on a surplus of food. While some queen breeders swear by this method and claim that it produces much more developed queens with larger ovaries, others say that it makes no real diffeerence.
You can try it and decide for yourself if the extra time and effort are worth the results you get. Of course, the second graft is maade only in cells that have been accepted, so there is a larger percentage of success with these graftings.