This is quite natural cat behaviour & you should now treat them as one litter. When my bengal queen had 8 kittens in her first litter, I supplemented her milk by giving cat formula (from the vet or pet shop) every 4 hours day & night to all the kittens. I weighed them all daily, keeping a record, which allowed me to notice if any were not getting their full rations! I also added the powdered formula to mum's food to make sure she didn't run short of any essential nutrients. I only had to feed the kittens for around 10 days & after 5 days was able to reduce the frequency to 4 times during the day. I offered mashed kitten food with added formula once their eyes had opened & they did wean quite early!
I also recommend speying your other girl, as she will return in season maybe as quickly as 1 month after she abandoned her kit! Entire cats that are allowed to just mate with strays are at extra risk of STdiseases & because they will wander more they are at higher risk of becoming a traffic casualty.
Using a properly kept stud cat costs quite a lot of money for good reasons! He has to be housed appropriately & is guaranteed to be disease free for his carefully, blood tested, selected 'brides'.
Incidentally, the kittens need to stay with mum until they are at least 8 weeks old, but better 10. My lot ate a box of 12 pouches of kitten food every day. Also recommended is not rehoming kittens until all have completed their course of innoculations at 12 to 13 weeks, minimum cost �35 each. Good luck finding homes for all the little darlings.