Sorry to hear about your other dog thats such a shame
i have hand reared two of my chihuahua litters and its hard but
if you already have your other chihuahua who is willing to help then i wouldnt be hand feeding aswell.
Every 1 / 2 hours i would put the pup on the back nipple and hold him so he can feed without falling off
if it is feeding even with help and putting on weight there is no need to keep topping it up unless the bitch doesnt have enough milk.
Have you hand reared a pup before? as if you dont do it right the milk could be going into it lungs and drowning it rattled breathing and gasping is a sign this is happening.
By the way your puppies and mum are lovely.