Do you want to compare one years month with the same month the previous year side by side?
You can concatenate the info in the dimension Month & '-' & Year with a sort expression
date(makedate(Year,Month),'YYYY/MM') so the order is right. If the bar chart is set to grouped it should show actual and budget side by side for each month.
if you use >= 2 active expressions and 2 dimensions, the values are always displayed in stacked bars.
(Please refer to the reference manuals, design properties for bar charts for more details).
If you want to see grouped expression values, one workaround would be to create a single dimension out of the two, like marcsliving said.