If knowing the genetic history of your offspring is important.....I wouldn't mix the 2 spawns....Plus 2 weeks apart the younger fry would most likely become food..
Make lot of water changes on that 5gal (I am sure you know this already)
One spawn is of a HMPK salamander dumbo ear pair and he other one is an HM salamander pair the female carries the big ear gene. My first spawn have hatched July sixth and I'm breeding my other pair this Saturday. Right now the current spawn is so small. Around 25 fry, I am feeding them vinegar eels, banana worms, and micro worms. And they are currently in a 5 gallon heated to 87
I start moving my Betta fry as early as 2 weeks of age-but my fry grow really fast-some wait until 4 weeks before they move their fry-I move the biggest first...I generally will move them by netting first-to place in a cup of their tank water to acclimate to the new tank water(I am sure you already know how to properly acclimate)
Mixing spawns-that depends.....if you can tell them apart or if it matters for genetic history reasons....plus size-if one spawn is much bigger than the other-the smaller fry may be eaten...
When I setup a new NPT the water is crystal clear when I am done....it is best to make back to back water changes until the water is clear-clear water is important for light penetration to the plants.
If the tank only has plants you don't have to worry about dechlorinator until the finial refill-plants can handle small amount of chlorine/chloramines short term. Just don't forget the dechlorinator on that last fill up....