Drag - airframe (all those extra bracing bits and interplane struts)
Drag - induced (twice as many wingtips)
Compression - interaction of airflow between the planes
Visibility - piss poor fwd vis as someone has left a wing in the way, poor rearwards as the pilot tends to sit low to overcome the fwd vis issues!
Biplanes, huh - I love 'em
I've flown the SEPECAT Jag, the Extra 300 and they were fun but I wouldn't swap my bipe for either!
Stik