Well that's a grey area we all know that it's fine on singles as long as you use Aberdeen tranny's and use doubles on handrails but you get some jumped up jobs worth saying it's no good etc that it's got to be done with towers and butts all on doubles etc(that uses so much more gear per lift and a extra fleet of boards to lap across the towers) or the new one is to use the klaws doubles which are designed for splays and scaffolds that use readylok arms(dog bones, animals etc),or if your old skool enough a dh on the bottom ledger and clip on top ledger (dh's are load bearing fittings 5.3kn)