My experience on the large moorland farm (lambing 520+ ewes each year) was that you could usually - but not always - get acceptance of the second lamb after 3-7 days of supporting feeds and giving the lambs access to a creep area so they could get to safety, but that a very high proportion of those "less favoured" lambs would end up pinching off other ewes and/or being completely abandoned a few weeks later. Some died and it was usually not possible to know if that was as the result of an underlying weakness detected by the mother or as a result of inadequate milk.
So my conclusion was that it was worth the effort of supporting feeds for approx 3 days but usually better to cut your losses and rear as a pet after that, and that any LFL (less favoured lambs) needed a mark to identify them as LFLs, and who their mum was, and a very good eye kept on them, both in terms of were they getting enough food, was the mum still mothering them properly, and were they pinching from other ewes.
I also thought, if the time was available, it was worth teaching them to take a bottle in the first few days. Then if they later had to be removed for their own safety or health, or to preserve the health and milk supply of the ewes they were pinching off, they would usually take to the bottle without issue. Whereas it can be a bit of a challenge getting older lambs to take to a bottle if they have never used one before.
I know some people say you shouldn't bottle feed a lamb as it will stop it feeding off its own mum, but I have come across this exactly one time only in 15 years, nearly 7,000 lambs, of which something like 300-400 would have had at least one bottle feed.