This data is linked on postcode and the crime details are at the level of postcode. Your post mentions parameters but there are no parameters associated with either of these data sources but instead two filters for date ranges. The date range filters are limiting the available data to make the views. I'm wondering if either of the data images could be represented as static data in a picture/image so the other data could be dynamic?
Thank you for sharing the workbook example to hep the community to see the issue.
A Tableau data blend is like a post-aggregate left join. So even though we have two separate data sources *filters apply to both*, just like they would if we'd done a join of the sources.
The workaround in this case is not to blend or join, but instead to union, add the range filters, and configure the range filters to keep Null values (from the other side of the union). Since these are two different files you'll probably need to do the full outer join on 0=1 technique covered in this post Dual Axis Mapping - Many Ways. Then you can add the two range filters & keep Nulls technique covered in this post Creating a Dynamic Range “Parameter” in Tableau | Drawing with Numbers.