Here's the comparison between two-handed weapon and two-weapon fighting for Warlock with Pact of the Blade, Thirsting Blade for extra attack, and Lifedrinker, at 5th level with a +1 magic weapon, and an ability modifier of +4:
+1 Two-handed greatsword or maul for 2d6 damage, at 5th level Warlock:
- With Hex: 2x (2d6 + 4 ability modifier +4 Lifedrinker + 1 magic + d6 Hex ) x 0.65 = 25.4 average
- Without: 2x (2d6 + 4 ability modifier + 4 Lifedrinker +1 magic) x 0.65 = 20.8 average
Two-weapon fighting with +1 one-handed weapon doing d8 damage in main hand and regular weapon doing d6 damage off-hand, at 5th level Warlock:
- Main-hand:
- With Hex: 2x (d8 + 4 ability modifier +4 Lifedrinker + 1 magic + d6 Hex ) x 0.65 = 22.1 average
- Without: 2x (d8 + 4 ability modifier + 4 Lifedrinker +1 magic) x 0.65 = 17.6 average
- Off-hand
- With Hex: 2x (d6 + d6 Hex ) x 0.6 = 8.4 average
- Without: 2x d6 x 0.6 = 4.2 average
- Damage by round with Hex
- Round 1: attack with main hand only, and cast Hex as a bonus action, average 22.1
- Round 2+: attack with both weapons, average 30.5= 22.1 + 8.4
- Until target drops and you need to switch targets for Hex or recast it as a bonus action
- Average across rounds is 26.3 for 2 rounds, 27.7 for 3, 28.4 for 4, 28.8 for 5
- Damage by round without Hex, assuming your weapons are both already drawn
- Round 1+: attack with both weapons, average 21.8 = 17.6 + 4.2
Two-weapon fighting deals more damage than wielding a two-handed weapon if your target lasts 2+ rounds. However, you have to sheathe the off-hand weapon to cast a spell with somatic or material components, then unsheathe it to two-weapon fight again next round. Alternatively, you take the Battle Caster feat so that you cast spells with V or V,S components with weapons in your hands; or the Improved Pact Weapon invocation from 5E D&D Xanathar's Guide to Everything to have your pact weapon as a spell focus to cast spells with V, or V,M or V,S,M components.
In a typical combat, I think the benefits for a Warlock of two-weapon fighting over using a two-handed weapon are minor, unless you really stack on the bonus damage at Warlock level 9+ with a spell level 5 slot (7 spell points) casting of the 5E D&D spell Spirit Shroud for +2d8 damage per hit. However at Warlock level 9 with Agonizing Blast and +5 spellcasting ability modifier, the invocations Eldritch Prism, Eldritch Severance, and Eldritch Squall do 4x d10 + 5 = 27 average force damage, save for half, across multiple targets for only 5 spell points; or cast Flame Strike to do 28 average (half fire, half radiant), save for half, across multiple targets for 7 spell points. So somewhat doubtful that casting the higher level Spirit Shroud is really worth it.