Y'know...I don't use any fancy VCOs, and I've been doing drone-type work since the mid-1980s.
The REAL key to making drone works isn't in the oscillators. It's in the everything else. You can take ONE single VCO and, by splitting the outputs from it off to different VCF/VCA chains, then using slow modulation to make that arrangement "cook", you'll arrive at much the same result. Then once you get your synth patch feed, try jamming it through a few different, paralleled outboard processors and mixing those alongside the "dry" synth signal.
Or, don't use VCOs AT ALL...instead, grab a shortwave radio, find something sonically interesting, then process the hell out of that. Or you could go to Alvin Lucier-style extremes and rig up a copy of his "Long Thin Wire" device. Or use a sampler and make small loops...if you edit the loop points "badly", you'll actually have something of a subliminal rhythmic pattern going on...which gets even more interesting if none of the loops are timed so that they have coincident points of repetition.
Everyone thinks that the way to do stuff like this can ONLY be via modular synths. It's not. Expand your electronic horizons a bit.