Level 6 warlock is able to maximize as well as empower their eldritch blast in separate rounds. If a warlock wanted to stack empower and maximize in single round, how would you calculate required caster level. Also, what would be the earliest level a warlock can cast empowered-maximized eldritch blast in a single round?
These requirements do not interact, you just need to meet each of them independently.
Unlike the Empower Spell and Maximize Spell feats, the spell-like ability versions don't do anything like change the effective spell level of the ability they modify. Instead, they have two requirements:
- You must be able to use a spell-like ability at caster level 6 or higher (prerequisite for the feats)
- The spell-like ability you want to modify must have a spell level equivalent less than or equal to your caster level / 2 (round down) - 2
So long as you meet these requirements, you may apply Empower, Maximize or both.
Per the Complete Arcane errata (.pdf download), Eldritch Blast has a spell level equivalent of 1. That means you can Empower or Maximize it starting at level 6.
If you find some way to qualify for the feats early, you could conceivably start throwing out Empowered + Maximized Eldritch Blasts as early as level 6. However, without early qualification cheese, the earliest you will be able to select both feats is level 9, since you would ordinarily select the first one as your level 6 feat and the second as your level 9 feat.