The double-weapon guess is reasonable. I don't know of any evidence, but the whole "absence of evidence is not evidence of absence" statement probably applies.
As for your war canoes, you're right, to a point. However, those long, thin paddled boats were typically used for short-distance fighting, as in Tahiti and New Zealand as well as in the Solomons.
Other islands quite certainly did use double-hulled warships. The most famous is the Fijian ndrua. Additionally, the long-distance raids I described in the
shark-toothed weapons thread had to have been carried out with voyaging canoes. Additionally, on the atolls, I don't think they had any purpose-built war canoes. If you wanted to attack a distant island, you voyaged to it.
Best,
F