I'm sorry, the question makes no sense because transmit and receive are interchanged.

I want to play Main and Layer part on a external Keyboard. Is that possible and how?

This depends on your external keyboard controller. As you seem to already know, although you state it backwards. The MAIN Part "receives" on MIDI Channel 1, and the LAYER Part "receives" on MIDI Channel 2. These receive channels are fixed. So in order to play, both sounds via an external controller you would need to transmit on two MIDI Channels simultaneously from that controller. Sorry, but we don't know anything about your external keyboard controller's capabilities. If your external controller can transmit on two channels simultaneously, then, yes you can. If not, then no.

The receive channels of the CP4 Stage are fixed.

But I want Main and Layer to transmit on same channel so i can play for example a piano layered with a pad on external keyboard (Maudio Keystation 88).

You are mixing up "transmit" and "receive". In a situation where your external controller is transmitting, the CP4 Stage is the receiving device.
MAIN receives on Ch 1
LAYER receives on Ch 2
SPLIT receives on Ch 3

We have to ask ...does your external keyboard have a natural wooden key, graded hammer action with triple sensors? Probably not! We cannot figure out why you would want to do what you're asking (not that we have to)... But seems strange.

The CP4 Stage has a superior piano weighted action, superior response (the triple sensors allow this action to "play" like an acoustic grand piano**), natural wood keys which feel the same as an acoustic grand, why do you need to layer two sounds on the same Midi channel? That is NOT how the engine of the CP4 Stage works.

** most dedicated controllers feature an action that mimics an upright piano, the CP4 Stage is designed to behave like a concert grand piano... The key does not have to return to full rest position in order to trigger the next note-on.

You can layer the two sounds on the CP4 Stage and play them both with the CP4 Stage's superior action. In order to accomplish this via MIDI IN... It will depend on your external controller:

If your external controller can transmit on multiple MIDI Channels then, and only then, can you play the MAIN and LAYER via MIDI IN, simultaneously.

Okay. The piano/pad example was a bad one. I love the keyboard. I want to play Layered sounds on my external keyboard. So i can for example play piano on the Cp4 and an Oberheim brass layered with a bell pad on the external keyboard. Did that make sense? Do you have any experience with M audio key station 88 es?

Okay. The piano/pad example was a bad one. I love the keyboard. I want to play Layered sounds on my external keyboard. So i can for example play piano on the Cp4 and an Oberheim brass layered with a bell pad on the external keyboard. Did that make sense? Do you have any experience with M audio key station 88 es?

No, as I mentioned in the previous post, we have no idea or experience with your external controller, but it does not have sounds or wooden keys, or triple sensors (of that we are fairly certain). The CP4 Stage can certain play the Piano as the MAIN Part, but where are the "Oberheim brass layered with a bell pad" coming from?

One of those 2 sounds can be assigned as the LAYER Part on the CP4 - where do you intend to get the third sound? Does the key station ES have sounds?

You dont understand. The Cp4 is amazing. I just want to play two voices from Cp4 on a external keyboard. Is that possible. You can use the SPLIT part as the piano sound (setting the split point on the highest note) and then use Main and Layer part on a external keyboard. To accomplish this the cp4 has to receive main and layer on the same channel and the external keyboard have to send on the same channel. Is this possible? Or is it fixed no matter how? I love the cp4, its the best keyboard i have ever played. I just want to use its voices/sounds on a external keyboard while playing the amazing piano on the Cp4 keyboard.

You dont understand. The Cp4 is amazing. I just want to play two voices from Cp4 on a external keyboard. Is that possible.

Yes. If that external keyboard controller can transmit on two MIDI Channels simultaneously.
MAIN Part receives on Channel 1
LAYER Part receives on Channel 2
SPLIT Part receives on Channel 3
(page 47, CP4 Stage Owner's Manual).

You can use the SPLIT part as the piano sound (setting the split point on the highest note) and then use Main and Layer part on a external keyboard.

Doing so will successfully prevent the CP4 Stage from playing the MAIN and LAYER Parts. Then you can play the MAIN Part by transmitting from your external keyboard on MIDI Channel 1, and you can play the LAYER Part by transmitting from your external keyboard on MIDI Channel 2. If you want both the MAIN and LAYER to sound simultaneously from an external controller you will need to transmit IN on both channels 1 *and* 2.

To accomplish this the cp4 has to receive main and layer on the same channel and the external keyboard have to send on the same channel. Is this possible?

No.

Or is it fixed no matter how?

Once again, yes, it is fixed.

I do understand. No, it will not work, *unless* your external controller can transmit on two MIDI Channels - this is not unusual for even basic MIDI Controller Keyboards, in fact, it is pretty standard... Does your external controller have this capability?
If not...

Alternate Method
Are you using this with a computer?
Perhaps if your controller only sends on a single MIDI Channel, you can use software to reChannelize its signal. Connect the external controller to the computer and reroute signal from there. For example, with a MIDI Utility or a DAW software (like Cubase), you can activate two MIDI tracks. Set each one to receive MIDI data IN from your external controller. Set the MIDI OUT to arrive at the CP4 Stage. Set the first Track's OUT CH = 1 (MAIN), and the second Track's OUT CH = 2 (LAYER).

Now when you select (highlight) both Tracks your external Controller can now transmit to both the Main and the Layer Parts of the CP4 Stage... While you play the 'Note Limited' SPLIT Piano Part across the entire NW, triple sensor KEYBED action of the CP4 Stage. This will work!
🙂
Hope that helps.