I may be completely wrong...
But if she is thin and not eating 100% of the amount she used to, then yes she may have less milk, and as a result she may only rear one lamb, but this may change if she starts to pick up
Is she on a lifeline bucket??
I suppose you could add milk powder and a bottle to your kit incase you have the rear the other lamb!
What tup has she gone to?
Glad things are looking up with all the sheep
hope you are alright!