This is annoying. "COUNTER-rotating" is the term you want.
"CONTRA-rotating" is when you have two different blades on the same
shaft spinning in different directions. About 50% of the time one of
these terms gets used, they get them mixed up.

http://en.wikipedia.org/wiki/Contra-rotating_propellers
http://en.wikipedia.org/wiki/Counter...ing_propellers

Chris W wrote:
The previous post on this topic got me wondering. Just how different
would the two engines need to be? I'm no expert, but it seems to me
cam shaft(s), magnetos, and prop are the parts that would be different
from one side to the other. I almost forgot, reverse the polarity on
the starter motor. What am I missing?