Regarding Condensation -
When the conditions are ripe you Will get condensation, whether you are under a "single wall" or a "double wall". A high open single wall may promote better circulation, but then you are compromising your "shelter effect" and getting wet from blown in rain. Many double wall flies are tight to the tent, close to the ground, and lack a vent, greatly enhancing the accumulation of condensate. The air flow (if there is any) can reduce the condensation, but again, it is not a function of single or double.
How you deal with the condensation is the issue. If you can control your movements under a single wall and not bump into the wet wall, all will be OK (given a good design and a tight pitch). All a double wall provides is a "bumper" to remind you to "go no further" (and maybe reduced heat loss).
When considering one over the other, remember to factor in proximity. Many single walls are small and low volume to minimize weight. Double walls usually have more volume and keep you further from the wet wall. But a reverse of this provides insight: I imagine a Mountain Laurel Design's Trailstar would be much easier to live in than a Big Agnes' Fly Creek UL1.
In either case you will still (occasionally) have to deal with a soaking wet, cold, sloppy, miserable piece of fabric. (And with a double wall, perhaps two.)