Well, you have to be inventive.
Are they completely identical? If there is anything like individual address or serial number, that's the seed. Nonvolatile memories (e.g. EEPROM) give much more options, of course. Any other input from outer word (a thermistor on ADC or so) makes miracles, but don't rely on it alone as a physical true random generator, unless you know exactly what you do. Sometimes it is recommended to use the LSB of an ADC for RNG but that might be disastrously wrong.

But in RNG/pRNG, ANYTHING may go wrong. There is no miraculous algorithm or randomness source; even the best one can be spoiled if incorrectly used.

But, I already repeat myself.

Jan Waclawek