How far apart should the wings of a biplane be to avoid aerodynamic interference? I am thinking about adding a lower wing to a high wing design to decrease wing loading for better 3D performance. Wing span will be 48 inches.

It's a largely sliding scale sort of thing. Common practice called for 1 to 1.5 times the chord but racing planes sometimes used as little as .7 to .8 of the max chord. Certainly at less than .7 bad things will become much more possible.

I wrote a long message a few mins ago telling about all the design parameters for bipes, but there was a posting error and I lost the message.

The long and short of it was:

Gap/Span is the important number, the bigger the better. You dont want to stagger the wings (can create unstable stall behavior, bad for a 3D model) and you want them to have the same angle of attack. You also want them to be the same length.

Otherwise, its a 3D aerobatic model, aerodynamics don't really matter much. Go with what looks right.

Oh yeah, one other thing. The aerodynamic center of a biplane is farther forward than a monoplane. Instead of being at 25% of the chord like a monoplane, it's more like 23%. So you will likely have to move your CG a bit forward to maintain static pitch stability.