The digital lines which control the SCXI portion of your combo chassis is shared between the PXI and SCXI portions (this is why you place the SCXI controller in the right-most slot).
But, you can't pass analog signals through this backplane. In order to read signals on your DMM with the SCXI-1129, they must be passed either from the front terminal block of the 1129 or from the rear of the module with the high voltage analog bus to the front of the DMM.
Hope this clears things up,
Jack