5kg of each is loads. great in the long run but if you want to try first, i would get smaller quantities if it's just for 2 babies.
1 kg of fiberfill is more than enough for 2 reborns. i've stuffed 3 and still got some left.
Personally, i use micro fine glass beads/granules for the limbs - 2 kg should be enough for 2 babies - i used glass beads in a sac in the body and head - 2-3kg for 2 babies? (all depends on size of baby)- i also use baby fat in bum and tum area (do not use in limbs or have contact with vinyl) maybe 2kg for 2 or more babies.
Sometimes i use both glass beads and polly pellets in the sac in the body, it just depends on the size and the weight you want to achieve.
Also, don't be too worried about finishing the doll and then taking it all apart again coz you're not happy with the weight or feel. i've done that!! it does take a bit of practice. i go through lots of cable ties coz i'm constantly filling, picking them up, squishing, weighing, undoing, refilling and so on!!
Just remember 1kg is 2.2lbs so try and think of a weight you want your baby to be and balance that weight into the limbs, head and body with all your ingredients (weighting stuff