You can pretty much always - 99% of the time - get a ewe to accept a single lamb.
Genuine success when she favours one lamb over the other is less common. I usually pen them for 2-3 days, then if she's still not letting the less favoured lamb suck, take it off.
However, a clever lamb will learn how and when to nip in and get a suck, so as long as mum has enough milk for two, you can make it work. I find it best to pen them together, provide a safe place for the lamb within the pen, support it for a feed at most twice a day - so it has to learn how to nip in and get sucks here and there between times. When you aren't needing to support it and it's not hungry, it's time to let them into a larger area. If the less-favoured lamb is still managing to stay full, mark it so you will know it and put them out.
Continue to monitor, because, in order of likelihood (in my experience) these are the potential outcomes:
- lamb does ok but is spotted 'pinching' off other ewes - augmenting the milk it gets from its own mum. If this compromises the other ewes and/or their lambs, you may need to take the less-favoured lamb off and reinstate it on the bottle
- lamb does ok for a while then starts to slip and is taken off and continued on the bottle. As the lambs' demands grow, she isn't producing enough for two; the less-favoured lamb ekes out a living on what it can get from her, plus grass, cake, and whatever else it can find - but isn't developed enough to really do well on limited milk, so fails to thrive.
- as above but failure to thrive isn't noticed, and lamb disappears or is found dead or dying
- both lambs reared to weaning, no issues