You need all the guards to be the same height. I do not think the double guards cut as well as a good single guard with a good ledger plate. With good ledger plates and a under serrated section they will cut as good at they ever could.
The biggest draw back to the double type guard is that they do not have a ledger/wear plate. So the bottom of the slot wears and you keep adjusting the sickle down to it. Then you break a guard and the replacement is not worn. So now you have a taller guard in the mix. You have to allow the sickle to ride the high guard. So then the rest are too loose.