Sometimes, if the ewe has limited milk, she favours one lamb and lets the other have the dregs - which ensures that one lamb thrives, so you can see why she does this.
If this is a factor here, then get her onto good grass, make sure she has ample cake, offer the ram lamb top-ups once or twice a day, supply lamb creep feed if you can from about 10 days onwards. Quite often, the less favoured lamb stops wanting top-ups after a day or two with mum on good grass.