If I started with a blanc slate, and empty preset, is it possible for me to layer this and have the independent crossfade control - velocity on the short and mod wheel on the long, while layering both patches?
Yes.
Start with a clean Synchron slate.
Click on the upper right gear icon, go to Engine, and unselect "Enable slots on MIDI activity."
In Perform, set Vel XF to CC1 (mod wheel).
Now drag the trumpet sustain "14 Trp1 Long Vib" into the first slot.
Beneath it, drag in the portato "05 Trp1 Portato"into the second (bottom) slot.
Click on the blue "Parallel" icon (with the stacked squares). Now you're in parallel mode.
Go to the Edit tab.
Click on the top slot (sustain trumpet).
Set "Parallel Mode" in the Options list to the lower right to Mix. (Now the sustain will continue to sound.)
Click on the bottom slot (portato).
Set "Parallel Mode" in the Options list to the lower right to XFade. (Now the portato will only sound per the dimension setting -- see next step).
Select the dimension box (to the left of the Parallel icon (blue stacked square icon).
Set it to Dim.Ctrl/A (though any dimension can work).
Now in the Dimension menu below the slots, click-hold the drop-down menu and select "Velocity." Now "Dim.Ctrl/A will respond to Velocity.
Play the keyboard and ride the mod wheel.
You should hear the sustain playing throughout, getting softer or louder depending the mod wheel, and the portato speaking softer or louder depending on how hard you hit the keyboard.
Okay, because "Enable slots on MIDI activity" is off, try this: disable the sustain patch (top slot) by clicking on the left portion of the slot.
Play again. The sustain is not playing, and you should hear the portato coming from near-nothing to the loudest amplitude (and strongest timbre) the harder you hit the keyboard. This helps to understand what's happening.
Going a little deeper in the weeds, you can adjust either slot's ADSR and volume in the Edit tab for a better balance and behavior.