Hello Travis,
Since you have multiple data sources in your report you can merge the dimension Last name and based on that Dimension you can create a Input control.
Now since you have merged the dimension of all the data sources you will have all the data sources affected with only one Input Control.
Please refer to the link below links on Merge Dimension.
Merge Dimensions and extended merge in Webi
Web Intelligence 4.0: Merge dimensions from different universes
Regards
Niraj