Peter,
Principally both mics can be used with the same power supply, as both use the same tube. However, and this is big:[ current draw may be so different between the tubes that you may under- or worse, over-supply one of the tubes.
AC 701s are known to be finnicky and get easily damaged when hit too hard by heater juice.
Therefore, before you plug any mic into a power supply desigend for an AC701, hook up a DC voltmeter to the power supply's mic cable connectors to measure heater voltage: Turn the mic on, and don't let the heater voltage go past 4.00 Volts.
Ideally I want to calibrate to ca. 3.90VDC- 3.95VDC at the tube itself (by measuring directly at the tube filament wire, with the mic's housing off, then adjusting with the various forms of trimmers found in power supplies.)
Adjusting to ca. 4.00VDC at the power supply will roughly get you in the ball park without having to worry that the tube will be fried.
Regarding the measurement input (pin #6) on the KM56:
You can conveniently leave it open, i.e. unterminated.
I tend to shorten the calibration feed in most LD Neumann mics like the M49 to ground, but that's a different circuity than on your KM.
Kind regards,