0.7 is pretty much as close as I'd want to go. There's no definite rule and bipes with as little as a .5 separation have flown but with tight spacings comes mucho drag and steep glide angles. Depending on what you're doing this may be acceptable. But with tighter spacings there is also a loss of wing effectiveness to the point where a 1000 sq inch total may only act like it's a 600 to 700 sq inch. Keeping the spacing up around 1.0 to 1.3 minimizes these effects.

But it's tough if you are doing a fun fly type. But with those the extra drag will create a nice constant speed sort of effect and will help duct the air during knife edge. In short you can get away with pretty much anything when the model is super light and has a honkin' big engine.