The sound obtained from oscillator sync is essentially a harmonic enrichment. This new version of the mod uses the 555 pin 3 output to reset the other oscillator via its reset input. Doing it this way guarantees a reset no matter where the synched oscillator is in it's cycle.

My circuit shows VCO2 syncing VCO1. VCO2 Pin 3 in normally high. This holds VCO1's pin 4 (reset input) high most of the time and lets VCO1 run free until a low pulse comes from pin 3 of VCO2. At that time, VCO1 is forced to reset no matter where it is in it's sawtooth cycle. Because of the way the offset control works, the effect only becomes audible after offset is set more clockwise than half way. After that, crazy harmonic changes start. I got clarinetish, bassoonish, metallic, buzzy and just plain bizarro synth sounds.

Some construction info:
First, pin 4 of IC15 must be disconnected from ground, then the above circuit can be connected. Pin 3 isn't used by the FatMan and floats. My ICM7555 (CMOS 555) ICs are in sockets. I also had some socket strips that I cut into two single pin sockets. I pulled IC15, bent pin 4 out and used a single pin socket to connect the wire to it. Same thing for pin 3 of IC16.