I plan to build a dual oscillator and am wondering if using a 556 rather than 2x 555 would be a mistake, ie oscillators locking to easily.

thanks

Bill

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=238

The 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).

I've found that my 3rd oscillator will NOT sync to the first pair no matter how hard I try. I used a regular 555 and an LED instead of the 10 ohm resistor.

the power cable to this board is about 18" long and I suspect that is the reason for this. Cable capacitance? has anyone tried a smaller power cap on the 555? maybe 1uf instead of 100 ?

Perfect! A new anti-phase-lock method! Use long wires between the main board and the VCO circuit-sections!

If you want them to lock and not beat, try moving the power supply wires to nodes local to the two VCO circuit-sections on the main board.