Parameters are workbook specific so they are able to cross Data Sources with the calculated fields. If you do not have a relationship established between your data sources then you will need to build out your views on two separate worksheets and then combine them onto a dashboard with the parameters. You mentioned that you are putting the calculated field in the filter section. If you are doing this, the filter will only be applied to the worksheet that shares a data source. If this is the case, then you would be better to build the parameters into the calculation instead of building them into a filter. For example: If you have a parameter that choose a facility and then two data sources that have a field [FAC1] and [FAC2] respectively (not identical for example purposes), if you build a calculation that states [Facility]=[FAC1] it would only be true for Data Source 1 and null for DataSource 2. If you want to filter off of the parameter it would be better to do something like this:
Data Source 1:
If [FAC]=[Facility] then sum([Number of Records]) end
Data Source 2:
If [FAC2]=[Facility] then sum(Number of Records]) end
This way the Parameter [Facility] is independently both Data Sources. If you have a workbook to attach we could troubleshoot more specifically to your situation.