The specific example I'm looking at here is: you are a Paladin 2/Warlock 5 and snagged Eldritch Smite from XGTE as your 5th level Warlock Invocation.
This gives you access to both…
Divine Smite
Starting at 2nd level, when you hit a creature with a melee weapon attack, you can expend one spell slot to deal radiant damage to the target, in addition to the weapon’s damage. The extra damage is 2d8 for a 1st-level spell slot, plus 1d8 for each spell level higher than 1st, to a maximum of 5d8. The damage increases by 1d8 if the target is an undead or a fiend.
and
Eldritch Smite
Prerequisite: 5th level, Pact of the Blade feature
Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller.
When I hit a target with an attack, can I set them both off at the same time, letting me burn two spell slots to use both features to boost my damage?
As was specified here, Divine Smite is not treated as a spellcasting, so it doesn't count towards the 'One Spell Per Round' rule. Presumably, neither would Eldritch Smite.
As far as I can tell, this appears to be a legal way to burn a ton of resources at once to generate a massive burst of damage. Am I correct? Can these two like-features be used simultaneously?
Best Answer
Yes, they stack perfectly well.
Neither ability requires an action to use, so as long as you hit with a melee weapon attack that happens to use your pact weapon you can use both. The only exception is that Eldritch Smite can only be used once per turn, while Divine Smite can be used as many times as you have melee weapon attacks.
If the intent was to use only one at a time it would be called out in the text or require a bonus action or something else that is clearly not the case here.