but I was UPGRADING an old project with two micro placed ad a few mm each other on the same PCB and that unfortunately used the same clock source.

one (bad) try that might save you:
If the uCs are not identical, try making the other the clock source (cut and wire the XTAL1 to XTAL2 connection).

a suggestion:
glue a plastic case oscillator to your PCB and wire it in.

a recommendation:
if possible redo the circuit and declare the upgrade a no go with current hardware

you also can:
get the older chip from a partminer (google on partnumber and the word "price"), a friend of mine still use the 751 which has not been made for who knows how many years (I just finished his new SMT design) he get the 751s from "partminers".

Erik