I always use split reins with a pelham/gag or whatever. IMO this is the most effective way to use them as you can differentiate between the two reins i.e ride mainly on the top rein and only use the curb for leverage when you need to.
Split reins arnt allowed in the Pony Club (yet another stupid PC rule, can anyone explain???). I think they look better and they are popular with small children who cant cope with doubles for showing.
I can not see how you can differentiate between reins on splits? The balance between the reins surely should be equal?
I had them to do Working Hunters on my 14hh pony as I could never have coped with double reins aged 10 as they looked smarter than roundings. We had to get the saddlery to do them specially.