I took the transfer function between H1:OMC-DCPD_A_OUT and H1:OMC_DCPD_B_OUT while the IFO was still at RF Readout.

The balance of the PD at DC is actually very good (delta<0.5%) but they are not well matched between 10-100Hz.
The difference is as much as 20%. The noise at high frequency is due to shotnoise between two PDs.

This could be caused by a mismatch between the V2cnt filters in the filter modules and the actual preamplifier filters in the vacuum chamber.
I'll look into this issue in the daytime as I don't want to screw up the overall calibration for DARM.