Actually, I find it easier to add a second lamb onto a first-timer than onto an experienced ewe.
But I totally agree with ZaktheLad, a first-timer making a good job of rearing a good strong single lamb is a much better prospect than the likely alternative, being an inexperienced young ewe with insufficient milk for two getting pulled down, possibly even getting mastitis, and making a poor job of the pair.
Having said which... if it's a very milky breed (like a Blue-faced Leicester, for instance), then there is maybe an argument for setting another lamb on.
It's not something I would be buying in orphan lambs to do, personally - and in fact we wouldn't even try to adopt our own orphan lambs onto a first-timer with a single unless she seemed to have an enormous bag and be very very fit, we were overrun with orphans, and there was masses and masses of lush spring grass for her to be eating.