In the old days we used to employ a Mach Zender interferometer to impose phase modulations at two different frequencies without creating the cross-term-sidebands between these two phase modulations. This was because we had used the double demodulation technique to sense the interferometer length DOFs.
Here are the three Pockels cells in the parallel paths of the Mach Zender.
Schematic
Attached below is the MZ schematic initial release. This doesn't reflect any real board though.