The explanation of VST Expression “Groups” is more than a little vague… in fact they only serve to prioritize which VST Expression Slots are called up (and are really only useful when used in the Score Editor… I’ll elaborate a little later 
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The important thing to realize, is that Cubase can transmit only one Slot at a time.
So, to have a Remote Key trigger two (or more) Articulations simultaneously, you must create a Slot with both the required key triggers contained in the same slot (upper-right section of the VST Expression Editor… use the “+” to create additional triggers)… and (this is the important bit )… when playing live (or/and recording)… that slot must have its own… single… remote trigger note (center section of the VST Expression Editor… not to be confused with the two trigger notes that are then transmitted by that slot).
For info, the way the Groups work in the Score Editor is as follows…
When a note has both the Articulation symbols present, it calls up that slot (which has one of the articulations in the Grp #1 column, and the other one in the Grp#2 column)… but that amounts to exactly the same thing as triggering that slot by playing the (single) remote key that you defined in the center section of the VST Expression Editor.
(Please don’t shoot the messenger 
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