Sally, I lamb between 2,000 and 3,000 ewes a year, and have done so for 20 years, never had a problem with putting a collie in front of a ewe and her lambs. Done it with blackies and swales, not a bother on the dog, may get a butt here and there, but thats the life of a working dog. Where people make a mistake is using the dog trick to get a ewe to take a set on lamb.
Back to the twins, I would suggest taking one off and bottle rearing it and then the other will prob get enough milk. Well done on your persistence and on getting them to day 3, be proud of what you have done.