What is the drawback of having a dual wing layout like a traditional Biplane?
If wing length is the issue, then why not put half the wing directly on top of the other half, possibly joining the two wingtips to reduce the vortex effects?
Two wings means half the aspect ratio per unit wing area = double the induced drag (roughly 25% increase in total cruise drag for fully-optimised cruise). kCl^2/2PiAr and all that jazz. This is the issue that all these tandem-wing concepts seem to overlook.
An "enclosed" wing tip doesn't eliminate the vortices, it just moves them to different locations - the drag remains the same. Again, a common fallacy not supported by data or analysis.
PDR