I understand your point about not feeding if not scanned, SteveHants - but there is a proviso. A ewe carrying twins may well need extra sugars and if she doesn't get them, she could suffer malnourishment of herself and the lambs, may not be able to provide the milk the lambs need, etc.
In the primitives on the hill, they would rarely succeed in bringing twin foetuses to the point of parturition, which would take care of this aspect. I'm not sure I would advocate allowing early abortion of second foetuses in lowland situations... Although, having said which, it is our farming practice to do the opposite of flushing the ewes, specifically to encourage most ewes to have one good single lamb and make a good job of rearing it. But we do then try to take very good care of them in the final 8 weeks!
So, if you are not scanning, I would, for the last 6-8 weeks, provide the minerals in a sugary lick, so that at least the twin mums can get the sugars they need. I would also watch the ewes' condition like a hawk, and immediately seperate and feed anyone losing condition from about 8 weeks to go onwards, as it almost certainly would mean she's carrying twins. (Assuming they are correctly dosed for fluke, worms, etc, that is.) Similarly, anyone getting fat in the final 6 weeks with only grass and a sugary lick to go at probably needs to be eating less, so you could move her/them to leaner pickings.
I realise that with 2 in-lamb ewes you probably won't seperate the thin one - but hopefully you get what I mean.