Tried the dual solution but never got it working, was trying to present both sales amount and % of sales for several customers, so was working with more than 2 alternatives...also other limitations like timing and so on.
Found a much easier solution, simply copied the expression (sum of sales for period x, customer in dimension) so I had 2 identical, and made one of them "relative"...since the proportions are exactly the same, it worked like a charm. getting both sales figure and % of sales.
Not perfect when zooming in to the small slices as the % is the percentages of % shown (maybe this was an issue with the dual solution as well?), but the overall picture works.