You won't notice it as neons are very low waste producers.
it will take ages for your sponges to get mucked up with neons.
I would put the fluval 2 sponges in to the fluval three for now.
Remove the fluval 2 filter for now.
It will takes ages if you have both filter's running.
Once you have a mature tank you could then add the fluval 2 back, and swap a sponge from the fluval 3.
The fluval 3 will cycle faster as it should pull more water throw it.
it will take ages for your sponges to get mucked up with neons.
I would put the fluval 2 sponges in to the fluval three for now.
Remove the fluval 2 filter for now.
It will takes ages if you have both filter's running.
Once you have a mature tank you could then add the fluval 2 back, and swap a sponge from the fluval 3.
The fluval 3 will cycle faster as it should pull more water throw it.