I have, for a couple of years, fed my pregnant lowland ewes on best quality hay, high energy+protein rumevite feed block, (available at all times) and a rumevite Lifeline (for sheep) bucket. This bucket is meant to help against twin lamb disease in sheep and is very effective. I only give them concentrates (about half a pound a day)in the last 2 weeks. Since I changed to this system, from the traditional feeding concentrates for the last 8 weeks, I have only had to help out one lamb and that was enormous - about 20lbs. All my other ewes have lambed unaided, had plenty of milk, and 2 raised triplets with no problem.
So, what I'm saying is foobar, that I think your fatties will be ok without additional concentrate feeding. But as Sally says, they will need a source of additional energy in the last month especially. You can supply this by giving them a high energy feed block and/or bucket which they can help themseves to as they need it.