As far as I know, there is no way to adjust the distance between groups superimposed on themselves. However, there is a trick to get the desired affect. All you have to do is SSRS trick. In your request, you need to create empty bars without labels. Not knowing how your query is structured, you can do it something like this:
Select 'Group 1' as Group_Label
, 'Bar 1' as Bar_Label
, 10 as Value_1
, 20 as Value_2
, '1' as Bar_Sort
Union All
Select 'Group 2' as Group_Label
, 'Bar 1' as Bar_Label
, 15 as Value_1
, 30 as Value_2
, '2' as Bar_Sort
Union All
Select '' as Group_Label
, '' as Bar_Label
, 0 as Value_1
, 0 as Value_2
, 1.5 as Bar_Sort
Order by Bar_Sort
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