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.