AVR-Chat

As others have indicated, you want to go with signal and ground in a
pair.  Putting two signals on a pair will cause large crosstalk.
I don't see why this won't work, but be prepared to insert some series
resistance in the driving end, maybe 100 ohms or so, to cut down on
ringing.  The cable capacitance will be the limiting factor on speed.

And of course, a good solid ground connection between the systems is important.

good luck!

-- 
There is no computer problem which cannot be solved by proper
application of a sufficiently large hammer.