The serial number on the outside may be different, but the Mac requires that the serial number field reported in the EDID information be in the correct field, and be different, to distinguish TWIN displays from each other:
My suggestion that this was caused in TWIN displays by the serial numbers not being presented in the correct fields was discovered by a User, who posted fairly convincing results of serial numbers present, but not in the approved fields.
switchresx serial number mac 23
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I know full well what is happening. The serial number have nothing to do with the display order. My monitors are the same. If it sees a dead port, (display off) it will respond to the first one that comes up. This is the problem.
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