Welcome to Yamaha Synth!

"Simple"? Well, we are not so sure it is simple. 🙂 but it is certainly do-able!

The Yamaha Motif ES had a built-in USB MIDI interface for itself and one external device. You can make use of this.
The Ensoniq is not known to us but we will assume it is 20th century technology and requires some help in addressing the computer. (correct us if we are wrong)...
The Yamaha Motif ES did not include a built-in audio interface for computer, so it will need to connect to your Steinberg UR22 (2-in/2-out) audio interface. Fortunately, the Motif ES has a stereo A/D Input that can be mixed and routed to the ES output, so you will be able to send audio from the Ensoniq through the ES and out to the UR22 and into the computer.

Here's what we suggest:

Audio
Connect the audio left/right outputs of the Ensoniq to the A/D Input of the Motif ES.
Connect the audio left/right outputs of the Motif ES to the Steinberg UR22 (Yamaha Steinberg USB Driver necessary)
Connect the left/right outputs of the UR22 to your monitor speakers
Connect the UR22 to the computer via USB.

MIDI
Connect the 5-pin MIDI Out of the Ensoniq to the 5-pin MIDI In of the Motif ES
Connect the 5-pin MIDI Out of the Motif ES to the 5-pin MIDI In of the Ensoniq
Connect the Motif ES's USB 'ToHost' port to your computer (Yamaha USB-MIDI Driver necessary)
Set Motif ES's MIDI IN/OUT = USB
Set the ES to MIDI SYNC = external MIDI
Set Cubase to send Standard MIDI CLOCK OUT to "Yamaha Motif ES-1 port1"

You will need two drivers:
Audio Driver: Yamaha Steinberg USB Driver (UR22) v1.9.0 for Mac
MIDI Driver: Yamaha USB-MIDI Driver (Motif ES) v1.3.2 for Mac

That should complete your 'simple' setup. You will be able to record/playback both audio and MIDI from/to either keyboard. You should be able to use either set of keys to play either tone generator (operate with Local Control = Off).

While using the Motif ES as your MIDI interface the Motif ES will be identified as "Yamaha Motif ES-1 (port 1)", you can set a Thru Port # for the external device (the Ensoniq). This is handled on the ES's UTILITY > MIDI > OTHER screen.

Audio from the Ensoniq will have its own Part in the ES with the ability to apply Effects (depending what you are doing). You will need to setup the PART as the 17th Part of your ES (even though it is external) it's audio signal will be combined with the ES.

Hope that helps.

P.S. An alternative setup could have you connecting the MIDI IN/OUT of the Ensoniq to the Steinberg UR22 (forgot that it has a MIDI interface built-in as well). In fact, if you use the UR22 for the Ensoniq's MIDI I/O, it will be identified in your software by the Yamaha Steinberg USB driver. If you add another keyboard or module you could connect it there.

Thank you for your time and information, i really appreciate it.
Do i need to set up the Mac - Audio/Midi configuration / midi studio, or is this not necessary?

kind regards,
Steven

Not necessary.