George,

Funny you should ask.  I've carried two canoes on my Yakima rack
attached to the rain gutters many times.  All you have to do is get two
of the Yakima extension kits along with the rack set up itself and then
go to the hardware store and get four bolts, nuts and washers to
assemble them, two extension spacers on each side of each rack.  This
gives you plenty of clearance (enough to even open the skylight) and is
very strong.  You will still want to tie the canoe/s to the vehicle
bumpers front and back as well as to the rack for proper and safe
transport.

Consider the Yakima gunwale brackets too, they make loading easier and
provide side to side stability on the rack for a single canoe.  Won't
work for two however, not enough width.

I can send pix if you like.
--
Stuart MacMillan
Seattle

'84 Vanagon Westfalia
'65 MGB (Driven since 1969)
'74 MGB GT (Restoring)

Assisting on Restoration:
'72 MGB GT (Daughter's)
'64 MGB (Son's)

Parts cars:
'68 & '73 MGB, '67 MGB GT