The double-barreled wand isn’t a wand at all, merely a magical set of metallic tubes that allow a user to wield two wands against the same target. Any want can fit into the double-barreled holder. Once a wand is placed into the holder it will act and operated as expected, though all variations of how this device works is left to the DM to cast any final judgments. For example, a wand that produces cute kittens, paired with a wand of flame, could produce flaming kittens, kittens that breathe fire, kittens made of fire. If two wands of the same effects are placed together, then double the effect or length of the effect.