I'd 'interfere'. Even if 'surrogate mum' is happy to oblige, these puppies, by 6 weeks, need to be independent of both their mother, and any other bitch who allows them to 'comfort suckle'. Clearly they aren't quite as forward as perhaps some of your other litters were - mine, by 6 weeks, wouldn't be interested in doing that, even if that were possible. And eventually this other bitch could start lactating, which won't do her any good! I have to say my puppies at that age, weren't 'with' my adults, so I never saw this because of that. I'd try giving them some warm goats milk, perhaps mid-morning, mid-afternoon and last thing to see whether this helps this 'need'.
In this instance, with this litter, I'd not have them anywhere near any adults and concentrate on getting them properly weaned so in another couple of weeks, they will be ready for their new lives. Just my opinion,