[AleD]

Код:

procedure TForm1.TrackBar1Change(Sender: TObject);
Var
    I:Integer;
    Priority:TThreadPriority;
begin
     Priority:=tpLowest;
        For I:=0 To (Sender as tTrackBar).Position + 1
          Do inc(Priority) ;
            If Sender=TrackBar1
              Then Thread1.Priority:=Priority
              Else Thread2.Priority:=Priority;
end;

попробуй так

Код:

procedure TForm1.FormCreate(Sender: TObject);
begin
  Thread1.Priority := TThreadPriority(TrackBar1.Position); //max и min от 0 до 6
end;