Whichever way you go, select the external op-amps and resistors referencing the notes in the FatMan modification which helps prevent them from sounding as one.
FatMan VCO APL (anti-phase-lock)
viewtopic.php?f=15&t=238The photos of your other projects were on breadboards. You might realize improved results building twin, dual, triple,... FatMan vcos on separate perfboard sections with point to point wiring and parallel connect them to the power supplies with their own ground and voltage wiring (and going with 555s on each instead of one with a 556).