The rules for the spell Animate Dead specify that "A destroyed skeleton or zombie [animated with that spell] can’t be animated again [presumably via that spell]". The rules for the spell Create Undead merely specify needing a dead body. Could one kill a humanoid, cast Animate Dead on its remains to create a Skeleton, then when that Skeleton is slain cast Create Undead to raise a Morgh from the remains of the Skeleton? How about vice versa?
We'll just put some relevant quotes at the top here, pulled from the respective spells. From animate dead:
Targets: one or more corpses touched
A skeleton can be created only from a mostly intact corpse or skeleton.
A destroyed skeleton or zombie can’t be animated again.
And from create undead:
Target: one corpse
Animate dead clearly states that you need an intact corpse and that you can't reanimate a destroyed skeleton or zombie- that's presumably because, when you destroy a skeleton or zombie, you no longer have an intact corpse to work with. It's also possible this is due to negative energy limitations on corpses, or any number of flavorful causes- the end result is the same. Once your undead creature is destroyed, you are left with a pile of bone and/or flesh that can't be animated again with animate dead.
Create undead specifically cites animate dead and has the same target- it is reasonable to conclude that the target, like animate dead, must be intact, though it isn't explicitly stated. Regardless, so long as create undead operates as a more powerful version of animate dead, it operates by the same constraints unless otherwise noted. You could try arguing that without the explicit "no destroyed undead can be used to make more undead" line in the spell, you should be able to pull it off- but I doubt you'll get far with that argument.
You can be stopped even faster by the specialized requirements of some of the undead in create undead, as they specifically state they require a humanoid corpse... and destroyed skeletons would, at best, be an undead corpse.