Not a simple question, at all! Welcome to YamahaSynth!

You seem to want to have two Masters... You cannot, not at the same time. You have two synths, one can be the Master, the other the Slave. You say you are using the S90ES as Master everything works fine, then when you say you want to play the XF and want both instruments to sound, you have an issue. Well, yes, you will. The Slave keyboard cannot become the Master keyboard without some fundamental change to your routing situation.

This does not mean you can't accomplish this but it will require some changes. I'm in an airport awaiting a connection as I type this, so I may not finish this in time. But thinking about it you might simply need to press the [Master] button on the XF making it the Master for this one scenario, and for the same setup, exit the Master mode on the S90 ES, temporarily, by pressing the mode button you want the S90 ES to be in in this scenario.

Either way it is not simple, as only one can be the Master (at a time).

I'll have to try some scenarios out and get back to you... But know this: the TRANSMIT channel setting is meaningless once you are using MASTER mode with Zones. Once you activate a Zone the Utility mode's "KBD Transmit Channel" setting is ignored, the transmit channel is determined by the active Zone/Zones. You will actually be able to use this fact in solving your issue. This parameter is only for when the instrument in question is in Voice or Performance mode, and only for transmitting OUT via MIDI. When in Voice or Performance mode the instrument does not address itself via MIDI Channel... at all. The Transmit Channel is for external transmitting, only.

Set the KBD Transmit Channel = OFF in both instruments. This will allow you to prevent the instrument in question from transmitting OUT via MIDI when you exit Master mode. This way the keyboard with Master mode LED active will always be the Master. And the channel or channels will be determined by its Master Zone setup.

In Master mode the S90 ES can transmit on as many four midi channels (Zones), the Motif XF in Master mode can transmit on as many as eight (Zones). You will need to run Midi cables from OUT-to-IN from the ES to the XF, and from the XF to the ES.

Set up the Master mode in the XF to address the S90 ES as you require.
You can have the S90 ES set so that when you exit Master mode it goes to the program you require for its tone engine (be it Voice, Performance, or Multi).

When you finish with this setup, simply exit Master mode on the XF, press Master on the S90 ES and you're back to where you know how everything works.

A Voice can receive on just one MIDI channel; a Performance can receive on just a single MIDI channel.
Only in Multi/Mixing mode can either of your two synths receive on multiple channels simultaneously.

Hello and thank you for a good answer! Sorry for late replay - I've been busy traveling 🙂

I have actually alreade done what you proposed, namley setting up also the XF as Master. Switching to Master Mode on the XF of course solves the problem. But then another problem enters - sooner or later I'll forget to change modes at the beginning and/or end of that specific song.:D That's kind of the reason I've set up all songs as master programs in the first place 🙂

Questions:
1. Is it possible to change modes from Voice to Master on the XF FROM a S90ES Master program? (I don't want to add additional gear on stage)
2. You write "Only in Multi/Mixing mode can either of your two synths receive on multiple channels simultaneously." - Yes but it's not possible to use a Performance in a Multi/Mixing program? At least not on the S90ES. Had it been a voice I wanted to use this would be a viable option.

Would it help any if I switched and insted use the XF as master instead of the S90ES?

This is not a very big problem but I like to find solutions and "tinker about" to learn more about the gear I'm using.

I've got at pretty good grip on zones etc and also have dual Midi-cables between both boards - but I didn't actually know that once Zones is activated that the KBD Transmit is pointless so thank you for that nugget of information!

Rock on!

//Mr T

Questions:
1. Is it possible to change modes from Voice to Master on the XF FROM a S90ES Master program? (I don't want to add additional gear on stage)

The message to change modes is not a simple Channel message, but is in the category of Parameter Change messages and is handled by a System wide message (system exclusive) message.

2. You write "Only in Multi/Mixing mode can either of your two synths receive on multiple channels simultaneously." - Yes but it's not possible to use a Performance in a Multi/Mixing program? At least not on the S90ES. Had it been a voice I wanted to use this would be a viable option.

A VOICE receives on a single MIDI Channel, a PERFORMANCE is four VOICES on the same MIDI Channel. Again, the only mode that multiple Voices can be simultaneously addressed is MULTI mode on your S90 ES, and MIXING mode on your Motif XF.

Would it help any if I switched and insted use the XF as master instead of the S90ES?

No. There can be only one MASTER at a time. The XF is more flexible as a controller because you have realtime ON/OFF switches for internal and external Zones when you use [PERFORMANCE CONTROL] while in a Master Zone setup.

This is not a very big problem but I like to find solutions and "tinker about" to learn more about the gear I'm using.

I've got at pretty good grip on zones etc and also have dual Midi-cables between both boards - but I didn't actually know that once Zones is activated that the KBD Transmit is pointless so thank you for that nugget of information!

By setting the KBD Transmit OFF you can opt not to communicate OUT via MIDI by exiting Master mode. With the XF as the Master you can easily turn On and Off your active Zones.