Do both workbooks use the same primary data source? URL actions cannot be used on secondary data sources so this would be the first thing I'd check.
Not sure if this helps exactly, but an old trick we employed regarding url parameters is to create a "dummy" view on the "splash page" of a workbook... Prior to cross-source filtering.
So if I have a url target servername.com/views/ExampleWorkbook/SplashViewName?ClientID=ABCD
and I want to pass the ClientID filter to another source in the "Example Workbook," I just would ensure SplashViewName was a dashboard and that it had at least one view built from Source 1 and Source 2 within the workbook. And then make sure ClientID was included in the filters shelf applied with the scope of "All Views Using This Datasource." Even if a view built from Source 2 is just blank, it'll still "receive" the url parameter and allow it to be applied across all other views using that source.
So when you pass the url parameter to SplashViewName, it covers all sources in Example Workbook.
I have 6 data sources within Workbook2, one of them is the same as the one used in Workbook1. None of the sheets use a secondary datasource so there shouldn't be any issue there. Thanks for the advice though! Definitely something to watch out for.