If the need to find a "foster" mom comes it could work, but you need to keep the actual momma still nursing so she does not dry up before the other litter is born. We had mommies nurse litters of 6 puppies just fine, it all depends on the lines and the dogs themselves. It sounds like you know what you are doing and have a plan set out already. I am sure you only needed some reassurance, and if the needs arise, your plan is not bad, and i am sure if it does not work out you will find a way around it. I have a female who would foster anything... she loves being a mommy and had foster a litter for another female. There are bitches that once they are done with their own litter they are done and would not accept new puppies. You are the only person who can decide what is best for your little ones.
Blessings on your little ones and the ones who are to come !
XOXO
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