There are several papers describing ways to couple PLL (phase locked loop) ICs to create chaos. I have been sitting on a stack of them for years but found the actual circuits quite limited in a synth.
Eventually it clicked that the low-pass filter needed to be a bit fancier than a pot and a capacitor, so this version gets two Buchla style low pass gate/filters.
The circuit contains two PLL chips and two low pass filters. The PLLs are voltage controlled together, the filters are controlled individually.
Normally in a PLL circuit, the output of the phase detector goes thru a low pass stage and then the output of this is used to control the VCO. In this version, the low pass filter outputs are fed to the non-inverting input of an op amp for it's own VCO and the inverting input of an op amp for the other VCO. Maybe the pic describes it best, this is from Intermittent Chaos in a Mutually Coupled PLL’s
System (Shirahama et al), tho I have seen similar circuits in other papers.
The PLLs and filters are all vactrol controlled but the PCB is a unpatented NLC black-box design, so no need to buy any expensive vactrols.
PCB set = USD23
Panel = USD22
assembled = USD200
Build Guide on NLC wiki