yes, two dialogs is the output..
if you know the content of used data sources (which variables are inluded) then you can play a bit with the sequence, you could show first the self made calendar and then set the variables on data source and first then trigger the variable screen (which would be prefilled already with the dates), or even you could filter out those variables and not show them twice...
I understand the requirement - but as of the available releases, the only way is this as above (which I see). you can surely place an idea for this extension on idea place (SAP BusinessObjects Design Studio: Home).
Sorry, I am afraid there is nothing better for now.