If both configurations are in the same project, you can define a build configuration template that contains the shared parameter (i.e. variable). Both active configurations must then reference that template -- use the 'Associate with Template' action to accomplish this. The value of the shared parameter must be set in the template in order for the same value to be known to both configurations.
If you're trying to share build numbers (or other system-level parameters) between the configurations, there is a special method for that.