Both the 6432 and the 648 can send signals back to the MADI card. For a loopback, the 648 can handle it on the device itself using the matrix (in groups of 8 channels, ie. you can loop back Ch. 1-8 and Ch. 57-64, while all others are converted to ADAT). The 6432 cannot loop back by itself. It converts 64 channels from MADI to AES3 and from AES3 to MADI at the same time. If you want to loop back, you need to add a loopback adapter.
MADI is unidirectional, as you know. The 648, 6432 and HDSPe MADI are bidirectional. Therefore, we need a complete list of sources and drains (ADAT channels in and/or out, AES channels in and/or out) that you wish to achieve before we can explain how it's wired.

The connections I showed were BS ... sorry wasn't concentrated enough ...
I meant like this: HDSPe MADI -> 648 -> 6432 and back: 6432 (can I here just pass thru selected ch.?) -> MADI card. But that isn't going to work probably because I cannot set which channels from the 642 pass thru and which not.
Now it comes to my mind to think about this: HDSPe MADI ->6432 -> 648 and back: 648 -> MADI card, if there is a way to let the 648 pass thru channels form the 6432. I hpe I was now in the correct mental state ... ;-) thank you again ...

To describe my setup here, especially how it is synchronized, would take a lot of space ... ;-) What you need to know is that my Yamaha DM2000 is clock master and receives 48 channels form the MADI card via the 648 as ADAT. The rest of the 16 channels is going thru DD8s and/or DD4s into external equipment with AES/EBU like TC6000 etc., all clocked by Lake People clock distributors. I would like to circumvent the necessity to use so many ADI-DDs. When I set this up first the 6432 wasn't available if I remember correctly. The setup is much more complex and consists of an additional RME card and further external devices and I may streamline it all more but describing this would distract from my main question. I guess it all boils down to the question if the 648 is able to be set up passing certain MADI channels thru from the input (in my case the last 16) and use the others for regular ADAT inputs?
The background: the whole system is working very well but switching to 96 kHz can be cumbersome and error prone because of all the converters. The idea was to use MADI<> AES/EBU to get it all working more elegantly. Now I see I may open a can of worms for me if the setup is modified to deeply ...