2 Different Instruments, 1 Stave

I don't think hiding this in that long thread was good for others wondering about how to do this.

Here are the steps:

1. Change the Trombone to the instrument from version 3.5 or later
2. Change the sound of the Mute Trombone to tube in the mixer
3. Add staff text that assigns Voice 2 to the Mute Trombone sound
4. Enter the Tuba notes in voice 2 and trombone notes in voice 1
5. Change the Trombone's range in staff/parts properties if you only want to see black notes.

Detailed explanation of each step follows:

You can make the Tombone 3 & Tuba on the same staff work with proper playback. In a recent update (maybe 3.5 or a 3.4.x) a second channel for mute was added to trombones (note: type of mute is not specified!) As long as you do not write for a muted trombone, you can add a tuba channel to it. Note that muted trombones are rare in classical music especially before about 1920. So the answer is how do you do this?

1. First, you must currently add the trombone by using Choose Instruments in the New Score wizard or change the instrument in staff/part properties. The templates I looked at do not have the new trombone in them though I didn't look at all of them. To change instruments in staff/part properties, right click the staff, choose staff/part properties and find the "Change instruments" button. Click it, select Trombone, click OK a couple of times and you now have a trombone with a mute channel. I will return to the staff/parts properties window later.

2. Press F10 to open the inspector and see that the Right pointing arrow above the trombone's volume slider is now black rather than gray. Click it and the Open and Mute channels will be visible. Click on the name of the Mute channel (I recommend this so you don't accidentally move the volume slider). By default you will see the sound near the top left is set to a Trumpet Harmon Mute channel (depending on your sound font). Change this sound to Tuba and press F10 to close the mixer.

3. On the staff, on or before the first tuba note (the first beat of the staff works fine) add staff text (ctrl+t is the shortcut), I suggest that you make it say "Tuba" but it can be anything you want. For example, if the first beat says "Tbn solo" you can use that text if you want to. After you add the text and it is no longer in edit mode, right click the text and choose Staff Text Properties and you will get a window that show four lines that start 1 2 3 4 followed by a dropdown that says "Open." Click the 2 on one of the lines then click "Open" on the same line and change it to "Mute." Click OK and now all notes entered into voice 2 will sound like a tuba because you set the mute sound to the tuba in the mixer. If you entered text for the sole purpose of setting up voice 2 as the tuba I recommend that you make that text invisible by selecting it and pressing V.

4. I'm glad you chose those measures from that score to show as an example. There is a limitation that will make an exact copy of that score difficult but not impossible. You must enter all Tuba notes into voice 2. That means you cannot write them as chords as in your picture. The following is optional: What you can do is Click 2 & 3 in the staff text properties step to make both voices 2 & 3 sound like a tuba. Why? because when you put voice 2 notes on the staff they will default to stem down, voice 3 notes default to stem up and the presence of a second voice on the trombone staff in the same measure (be it notes or rests) will cause all voice 1 stems to point up. So if measure 1 had both trombone and tuba notes the stems will point in the directions I just said. If in measure 2 there are only voice 1 notes and rest, the stems will be automatically pointed in the default direction (up for D an below down for anything above D). So when you want tuba notes to be stem up, put them in voice 3, when you want them stem down, put them in voice 2. Otherwise you can always use voice 2 and the stems will always point down by default. If you want a tuba stem to point in the opposite direction, select the note and press X. If you want it to look like a chord and there are beams on you notes as in your example, select the beams that should be invisible and press V to hide them. There is one big advantage to only using voice 2 for the tuba. If you create parts, they will look nice by default when you tell the parts dialog to use voice 2 of trombone 3 for the tuba. If you use voices 2 & 3, you will need to manually adjust the part's stems and make extra rests invisible and tell the parts dialog that voices 2 & 3 are to be used for the tuba.

5. Now back to staff/part properties. When you enter the tuba notes, you will notice that everything below E2 will turn olive until you get to C2 when they turn red. This is because you have exceeded the trombone's range. I suggest that you set the bottom of both amateur and professional ranges to Bb0 so you'll have a good range for the tuba without the notes' colors changing. Click the pencil icon next to the note name and click the note in the window that pops up. This is optional but I don't like anything but black notes on my score.

There are other ways to do almost every step I have described but these are the methods that work for me.

Look, don't get me wrong. Combine voices if you want. I'm just wondering why this is important. Sure tenors and bases on one line work because in doesn't cause a problem with playback.
What if the source has an ambiguity in it? Do you just pass it on? If you are trying to pass on the source as it is, a score to be stored for posterity, that's one thing. I get it. But why does it matter if you are after playback. Which, by the way, isn't going to sound very musical unless you do all kinds of things to it.
Why is it disrespectful to the source to separate instruments to get playback.
Consider that the "source" was intended to be played by real musicians. Musicians that know how to do their job.
It was never intended to be realized artificially by a computer.