It usually self-regulates as they need more milk.
I have intervened in such cases on occasion, usually when the other teat is quite engorged and it will be a problem if they don't get on it quickly. Decide which lamb you want to use the less favoured teat (and use that lamb to do this all the time) and teach it to drink that side - pen it, when it's hungry, with the mum and not the sibling, and cover the favourite teat with your hand (with the fingers turned under) so that it can only get milk out of the other teat. Usually only takes 2 or 3 feeds for it to have switched to that teat.
If the teat is really engorged and too big for its mouth, you may need to milk some off until the teat is less engorged.