Just...  Uhhh...  To throw in 2 more cents worth...
Wilfy was presuming that two cylindrical copper pipes would be better than
a single oval cross sectional aluminum pipe. Based on the heat conductivity
of copper over aluminum and the increased surface that's obvious when two
circles are compared to a single oval of the same area it is very easy to
presume exactly what Ken did. Obviously Ken's not stupid. I'm not going to
do the math for you either, but I'll estimate the copper as a little less
than three times as efficient at heat transfer based on my vague memory of
the description.
If you have difficulty assuming that more heat transfer can occur across a
greater surface area, or don't understand the geometry involved even
basically, please either trust Ken's advice, he seems to presume things
well, or never work on your own vanagon. They're odd beasts and can suffer
nasty consequences if stupid things are done to them.
Rant mode off.