After fighting relentlessly with the mode cleaner, I believe I have achieved final results.
I have mostly been focusing on Wiener filtering MCL with a SISO Wiener filter for a reason, simplicity. This simplicity allowed me to understand the dificulties of getting a filter to work on the online system properly and to develope a systematic way of making this online Wiener filters. The next logical step after achieving my final SISO Wiener filter using the T240-X seismometer as witness for MCL (see elog:11535) and learning how to produce good conditioned Wiener filters was to give MISO Wiener filtering of MCL a try.
I tried performing some MISO filtering on MCL using the T240-X and T240-Y as witnesses but the procedure that I used to develope the Wiener filters did not work as well here. I made the decision to ditch it and use some of the training data I saved when the SISO (T240-X) filter was runing overnight to develope another SISO Wiener filter for MCL but this time using T240-Y as witness. I will compare how much more we gain when doing MISO Wiener filtering compared to just a bunch of SISO filtering in series, maybe a lot, or little.
I left both filters running overnight in order to get trainining data for arm and WFS yaw and pitch subtractions.
The SISO filters for MCL are shown below:

The theoretical FIR and IIR subtractions using the above filters:

Running the filters on the online system gave the following subtractions for MCL and YARM:


Comparing the subtractions using only the T240-X filter versus the T240-X and T240-Y:


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