Here's a silly thing to suggest
One can pick up a Danelectro Fab flanger for a pittance these days. Buying two of them is cheaper than buying almost any other individual flanger. Strikes me that figuring out where the dry-side mixing resistor is and removing it, and feeding a pair of them with a splitter, then taking their outputs to a 2-input mixer, would provide a pretty darn quick introduction to through zero flanging.
Note that this would provide limited access to both symmetrical and asymmetrical TZF. For latecomers, asymmetrical TZF involves having one very short fixed delay and a second swept one. The through-zero poin occurs as the swept delay gets shorter than what the fixed delay is set to. How "long" the sweep spends on the other side of zero depends on the sweep speed and depth, but also on what the fixed delay is set to. A symmetrical TZF involves having two counterswept delays driven by the same LFO, with the through-zero point being where they each achieve the identical time delay (one on the way up, the other on the way down).
Using a pair of Fab flangers provides speed, width and regen controls for each delay. The individual LFOs are not readily sync-able, although I suppose some invasive surgery could cure that, but I'm aiming for quick-and-dirty not lengthy-and-involved. By keeping the sweep rate VERY slow, and the sweep width minimal, that would permit one of the delays to be relatively constrained (although obviously it would change a bit). Increasing the sweep width equally on each pedal would provide something closer to symmetrical TZF, though clearly not as predictable as something like diyfreque's modded Dimension C.
Note that the switching scheme on such pedals almost always involves use of a single FET to connect or lift the delay signal from the mixing stage. So, if you remove the dry signal from the mixing stage, stepping on the pedal's bypass switch will get you stoney silence. The bottomline is that should one wish to use a pair of pedals in this manner for gigging or other performance contexts, I suspect your only choice would be to use the whole mess with a loop selector pedal. Still, you have to admit that a pair of $15 flangers, and $10 worth of mixer-splitter stuff and patch cables is pretty inexpensive.
