Well I do not think it is big enough and adding so many rocks and plants will take up so much room and you NEED another hiding place you are going to have a hard job keeping it clean and they might start fighting....is there no way you can have them in both tanks? it really would be better for them and for you.
As Axolotl stay on the floor most of the time the fact that this tank is a foot higher does not make a difference for the room your Axie's have it will help a little with water quality. the new tank only has one square foot more floor space that means they each have one and a half square feet.
It really is not big enough they are not going to be happy and may get ill.
I am going to ask a few questions see if you are doing everything right:
Can you tell me what they eat?
Do you understand cycling a tank and using de-chlorinater?
Have you got a water testing kit?
Do you have anything to clean the bottom of the tank, a syphon or gravel cleaner?
Do you know what temperature the tank is?
Are your axolotl's gills pointing straight and up towards the sky or curled and pointing forward?
Are there tails straight or curled forward at the tip?
Does the filter have a strong current? Axolotl live in ponds and lakes and hate current
they eat worms,bloodworm(frozen),maggots and coi pellets
cycling is the nitrogen cycle in a nut shell
dont use de chlorinator use ro water
dont have a test kit as of yet i am affraid
i have rubber tube that i siphon the **** off the bottom with
dont know how cold my water is i just know it is cold when i put my hand in but not too cold
my axys gills are pointing towards his tail and have been ever since i got them 3 week ago
and their tails are streight
and no the tank has pretty much no current , the water is moving but isnt like a river
Also what do you mean by 'activated charcoal' and with the river rocks you need to make sure they are MUCH bigger than your Axolotl head they will try to eat anything and can swallow larger things than you would expect!
Activated carbon is used in aquariums for chemical filtration look
HERE