Not sure what your question is... (?)
The device you are using as your control device (the one sending the Set List changes) needs to send data to both Synthesizers. Both synths are connected to the Set List device, Send the Bank Select and PC to each device from that Set List.

That’s how you have it set up. If you were using the MODX7 as the control device then it would send Bank Select and Program Change to the MOXF6. But you selected NOT to use the MODX7 as your control device.

Your question is really: how to address two keyboards from your selected control device (the one with your Set List)...

Not sure what your question is... (?)

Can I use the iPad as main controler ( only for the MODX7 ) and the MODX7 as secondary controler for the MOXF ( and perhaps virtual instruments on the iPad ) in one setup?
Has the kind of connection ( via USB-A port or USB-B Port ) influence on the way the connection works? A=sender /controler B=receiver

USB is send and receive. Since the device addressing is different between MODX and MOXF - they are seen as two unique devices by iPad without your having to change anything. If you had two MODX keyboards - then you would want to have two different device numbers - but that's not a consideration here.

Your two keyboards do not "see" each other. There is no communication from keyboard to keyboard. Notice the USB port on MODX and MOXF says "To Host". That means the communication will be to and from the host. The host, in this case, is the iPad.

Also be aware that MODX does not send out MSB/LSB/PC information (bank and program change) for preset performances. I do not think there is a global setting to force this on. The setting is contained within each Performance. Therefore, you need to turn this on yourself for all MODX Performances and save the User Performance (a new one, or update an existing one). This is probably one of those things that deserves a global setting. I think globally you can just filter out if MODX recognizes incoming bank/program changes.

If you want anything to go from the MODX through iPad to MOXF - you need to program the iPad to echo these messages over to MOXF. Most DAW software can handle this.

I think the iConnectivity iConnectMIDI2+ might let you do everything you want... it's a 2-in 2-out iOS MIDI interface with all kinds of routing options. Alternatively, you could do it with a combination of a regular computer MIDI-USB interface, the Apple Lightning to USB 3 Camera Adapter, and a combination of MIDI Merge and MIDI Thru boxes, but that would create a bit of an octopus of wiring. You might also be able to do it with the hardware setup you're using, if you can use an iOS app that will take MIDI in from one keyboard (your MODX7) and send it to the second keyboard (your MOXF). Possibilities might be iMIDIPatchbay, Keystage, MIDIflow, and the one coming at https://www.camelotpro.com/

Thanks Scott
Camelot Pro seems to be a very good idea... let’s wait