Hello Stefan,
Nested triplets are not (yet) allowed but you can use layering to achieve the effect you want so that it looks correct when you print it and it sounds correct when you play it.
Here is the example - you need to layer the two staves.
!NoteWorthyComposer(2.5)
|Locale|ACP:1252
|Editor|ActiveStaff:1|CaretIndex:1|CaretPos:2
|SongInfo|Title:""|Author:"<Name>"|Lyricist:""|Copyright1:"Copyright © 2012 <Name>"|Copyright2:"All Rights Reserved"
|PgSetup|StaffSize:16|Zoom:4|TitlePage:Y|JustifyVertically:Y|PrintSystemSepMark:N|ExtendLastSystem:N|DurationPadding:Y|PageNumbers:0|StaffLabels:None|BarNumbers:None|StartingBar:1
|Font|Style:StaffItalic|Typeface:"Times New Roman"|Size:10|Bold:Y|Italic:Y|CharSet:0
|Font|Style:StaffBold|Typeface:"Times New Roman"|Size:8|Bold:Y|Italic:N|CharSet:0
|Font|Style:StaffLyric|Typeface:"Times New Roman"|Size:7|Bold:N|Italic:N|CharSet:0
|Font|Style:PageTitleText|Typeface:"Times New Roman"|Size:24|Bold:Y|Italic:N|CharSet:0
|Font|Style:PageText|Typeface:"Times New Roman"|Size:12|Bold:N|Italic:N|CharSet:0
|Font|Style:PageSmallText|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User1|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User2|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User3|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User4|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User5|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|Font|Style:User6|Typeface:"Times New Roman"|Size:8|Bold:N|Italic:N|CharSet:0
|PgMargins|Left:1.27|Top:1.27|Right:1.27|Bottom:1.27|Mirror:N
|AddStaff|Name:"Staff"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:12|BoundaryBottom:12|Lines:5|WithNextStaff:Layer|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:1
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Rest|Dur:4th,Triplet=First|Opts:Stem=Up
|Note|Dur:4th,Triplet|Pos:-1|Opts:Stem=Up
|Note|Dur:4th,Triplet=End|Pos:-1|Opts:Stem=Up
|Note|Dur:8th,Triplet=First|Pos:-2^|Opts:Stem=Up|Visibility:Never
|Note|Dur:8th,Triplet|Pos:-2^|Opts:Stem=Up|Visibility:Never
|Note|Dur:8th,Triplet|Pos:-2|Opts:Stem=Up|Visibility:Never
|Note|Dur:8th,Triplet|Pos:-3^|Opts:Stem=Up,Beam=First|Visibility:Never
|Note|Dur:8th,Triplet|Pos:-3^|Opts:Stem=Up,StemLength=12,Beam|Visibility:Never
|Note|Dur:8th,Triplet|Pos:-3^|Opts:Stem=Up,Beam=End|Visibility:Never
|Note|Dur:8th,Triplet|Pos:-3|Opts:Stem=Up,StemLength=5,Beam=First
|Note|Dur:8th,Triplet|Pos:-4|Opts:Stem=Up,Beam
|Note|Dur:8th,Triplet=End|Pos:-2|Opts:Stem=Up,StemLength=6,Beam=End
|Bar
|AddStaff|Name:"Staff-1"|Group:"Standard"
|StaffProperties|EndingBar:Section Close|Visible:Y|BoundaryTop:10|BoundaryBottom:10|Lines:5|Color:Default
|StaffProperties|Muted:N|Volume:127|StereoPan:64|Device:0|Channel:2
|StaffInstrument|Trans:0|DynVel:10,30,45,60,75,92,108,127
|Rest|Dur:4th,Triplet=First|Opts:Stem=Up
|Note|Dur:4th,Triplet|Pos:-1|Opts:Stem=Up,Muted
|Note|Dur:4th,Triplet=End|Pos:-1|Opts:Stem=Up,Muted
|Note|Dur:4th|Pos:-2|Opts:Muted
|Note|Dur:4th|Pos:-3^|Opts:Muted
|Note|Dur:8th,Triplet=First|Pos:-3|Opts:Stem=Up,StemLength=5,Beam=First,Muted
|Note|Dur:8th,Triplet|Pos:-4|Opts:Stem=Up,Beam,Muted
|Note|Dur:8th,Triplet=End|Pos:-2|Opts:Stem=Up,StemLength=6,Beam=End,Muted
|Bar
!NoteWorthyComposer-End
<edit> Correction made </edit>