Since the DAQ card is in the last slot of the PXI chassis it will be the one controlling the switches. Although you can't control the switch with the DMM you can still use handshaking to scan the SCXI 1127 using the external triggers (with the mentioned AUX cable). To achieve this you will need to set up the Configure Scan Trigger to have both the triggers be External. Do the same for the DMM. Then externally connect the trigger lines to the following connections
DMM EXT Trigger <-------------- Switch Scan Advance
DMM VoltMeter Complete ---> Switch Trigger Input
The DMM EXT Trigger is the trigger that the DMM waits for this trigger input take a measurement.
The DMM Voltmeter Complete is the trigger that the DMM sends out when the measurement i
The Switch Scan Advance is the trigger that the switch sends out when the relay has settled.
The Switch Trigger Input is self-explanatory.
There is an excellent sample program on the Developer Zone that demonstrates this. Simply change the TTL lines to external and the instrument descriptors and you are ready to go. This can be found at Switch_SCANNER_With_DMM_Handshaking.vi