I have a 2004 Tacoma quad-cab with a Yakima rack. To carry two canoes, I worm-clamp a long cast-iron pipe with upturned ends (right angles and short bars) to each of the Yakima bars.

It works, as we've been carrying two-canoes the 930 miles (each way) from my house to Ely almost every year for a decade.

-- kypaddler