If you make a selection in a bar chart, you are not selecting the bar, but the underlying dimension, and AFAIR, if you are using two dimensions, you will first select on the first dimension, then on a second click on a segment / bar the second.
Hence, the time you fire your trigger, there is no information in the selection state that you clicked on a specific value in first and second dimension.
If you need both informations, I think you need to let the user make a second selection on second dimension, then create a trigger that checks that boths fields have selections in an open your other tab.
As swuehl said, First you select the Primary dimension value in chart, then let user to select the values of secondary dimension values as well. Now you can set the trigger on secondary dimension to go to next sheet so that both the dimension values will be selected on next sheet