**Yup.**

Both require attunement, but you've got three attunement "slots" so there's no problem there.
One requires the wielder be a Warlock, the other just requires the wielder be a spellcaster. So no problems there.

Looking elsewhere the only conflict we might run into would be "Multiple Items of the Same Kind" (DMG p.141). But that's directed at things--like layering two pair of gloves or boots--that would be physically incompatible. Holding two one-handed objects really doesn't seem to run afoul of it.

If this seems overpowered, just remember what it's "costing" the warlock: two of her three attuned items, along with whatever it took to get two *very rare* magic items, plus a free hand for material components. And it only benefits her spells which require attack rolls. It's a lot, but it's not worse than a rogue who got her hands on a *+3 longbow* and a *Manual of Quickness of Action*. The point is: if your game's got very rare magic in enough quantity for a single character to grab two harmonizing items, that's going to be a lot of power.