Let’s start from the definition of tremolo.
It refers to my opinion to a technique on guitar which was made (in the earlier days) by the amplifier which was interrupting the volume signal artificially at a certain continuous defined tempo. It could not recognize the start of a new note. Later we had synths which recognised the new note start which was very significant while playing.

Today I did not find any possibility to reproduce the Tremolo/Trill in the whole XS machine for this target.
I admit, the wishes are old fashioned, but technically I cannot afford that a Casio player does it without problems.
I am prepared to do a serious effort on this.
Problems :
I intend to make an Arp, but I do not know how to make this as I want to hear 2 single mandolins with no gun effects and all together synchrro.

In other words : 2 mandolin players play separate separate notes with “tremolo”
How do you do this with Arps ?

Tremolo you describe correctly - it is AMPLITUDE MODULATION. Change in Volume.

Trill involves multiple notes being triggered, Tremolo does not. So there is that big difference.

Tremolo is done in many ways on the Motif XS, (sorry if you could not find, them - it is not that they are not there, let’s be clear, it is that YOU could not find them, right?)

Tremolo which is a change in amplitude (or loudness) is simple loudness modulation.

This can be done by rapidly moving a controller sending cc011 (Expression) - this could be done with a foot pedal, or you could automate this with any physical controller of your choosing. Other methods include using the TREMOLO amplifier Effect (just as done on classic guitar recording of the past).

Examples to study:
“Middy Tremolo” - PRE2:098(G02) - great example of the Gtr Amplifier tremolo
Please check out the Tremolo - presets, here’s how:
Press [F6] EFFECT> to take the shortcut to the COMMON > Effect screen
Explore the INSERT A = Tremolo
Press [SF2] INS A - to view its parameters
Try the different ‘Preset’ tremolo settings to get a quick idea what can be accomplished with this Insert Effect.

“Tremolo Strings 1” - PRE4:026(B10) - great example of Tremolo being applied a Low Frequency Oscillator (LFO) using AM (Amplitude Modulation). Please check out Tremolo - here’s how:
Press [EDIT]
Press [COMMON EDIT]
Press [F4] CONTROL SET
Control Set 2 is applying Element LFO Amplitude Modulation via the MW
in Control Set 1 you can see that KNOB 1 is controlling how fast the LFO speed is offset
in Control Set 5 you can see that MW is also applying some Pitch Modulation as well.

Experiment with these two examples, of creating tremolo.. you will notice a non-synchronized tremolo happening - this, I’m sure, is on purpose. Navigate to the individual Element LFOs to view that each is at a slightly different speed - as would happen with real string players - this way as you intensify the tremolo there is a subtle difference in each Element

Press a numbered button [1]-[4] while in EDIT to view ELEMENT parameters
Press [F5] ELM LFO

I’m not exactly sure what you are looking to do, whether it is the guitar amp type tremolo, which can happen with held chords… or trills, which to me, require multiple notes being played. While I get the similarity, the resultant sound is so very different… I’m just not exactly sure what it is you are going after.

But these are just two methods of creating TREMOLO (which is the musical term for Amplitude Modulation via an LFO - which translates to rhythmic loudness modification, in common language).

Problems :
I intend to make an Arp, but I do not know how to make this as I want to hear 2 single mandolins with no gun effects and all together synchrro.

In other words : 2 mandolin players play separate separate notes with “tremolo”
How do you do this with Arps ?

Not quite sure what you mean you want to hear 2 single mandolins - do you have a VOICE that is 2 single mandolins?

An arp will not, necessarily, make a Voice that is 1 mandolin sound like 2 mandolins… so the question makes me think I’m missing something (of what you are thinking). Creating a trill is easy… play what you want the arp to do, and use the PUT TRACK TO ARP job to translate that to an arp…

Use the CONVERT TYPE = NORMAL

It should only sound like a machine gun if you play it like a machine gun… You can do some real magic with arpeggios in terms of manipulating velocity and gate time, but it is going to be true that the closer the source data is to the actual trill sound you are looking for the better and more useful the data will be as an arpeggio.

So really good use of velocity in the source phrase is going to be critical. Typically when trilling on plucked string instruments there is a sense of louder/softer and longer/shorter it is that information that will be critical to it sounding realistic.

You can create and sculpt the source phrase to your liking - one measure minimum is all you need to execute. Once that phrase is converted to an arpeggio it will play exactly as you require.

Now I did mention creating a simple trill is a matter of inputting two different alternating pitches as the source phrase (again velocity and duration is important). You can create a ghost of each slightly delayed to make it sound like a second mandolin is involved. Again the spacing between the two distinct picks will need to considered. For example, to make a six note chord sound strummed separating the notes by 006 clock ticks makes an obvious ‘strum’ type effect.

So to make it sound like two mandolins play a “double note”, one on the downbeat marker and the ghost about 006 clock ticks later. You will have to experiment because this is going to be affected by the tempo of the song the arp is going to be used in… and how intense you want the trilling to sound. The spacing between the two should not be overly obvious but should not be simultaneous - anything but simultaneous. I’m just suggesting the 006 clock tick distance because that’s approximately how many clock ticks pass between strings as a guitar tries to play a 6 note chord. This is why you don’t want to quantize guitar data, a guitar chord that is strummed low to high string takes about 024 clock ticks between evolve. A guitar strum is anything but six simultaneous notes actually the first string can easily be 24 clock ticks in front of the last string a quick strum. But when all six strings sound simultaneously, it does not sound realistic.

To sound like two guitars plucking strings simultaneously the “006 clock ticks” rule of thumb is pretty good, but again mileage will vary depending on what you are doing with it. But I just want you to be aware that separating two notes of the same pitch by a few clock ticks will sound like two instruments. You can experiment and adjust as your ear tells you…

Hope that helps.