Can a warlock hold a +3 Rod of the Pact Keeper in one hand and a +3 Wand of the War Mage in the other and gain +6 on her spell attacks?
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.
Actually, they do stack, but only under certain circumstances. There must be a material and somatic component to the spell, and the spell's component MUST be something that an arcane focus can replace. In addition, it must be a warlock SPELL cast, not just a spell cast from a warlock slot (although, it can be a warlock spell cast from another class slot, such as Sorc/Bard/Pally, if the character is multi-classed.