A data source object is consumed when used by a report/subreport/part, so you'll need separate objects for the two parts.You can do that by directly passing your list/collection of data objects instead of a JRBeanCollectionDataSource object, and creating fresh JRBeanCollectionDataSource instances from the list of data objects. As in
<field name="params" class="java.util.List"/>
...
<parameter name="REPORT_DATA_SOURCE">
<expression><![CDATA[new net.sf.jasperreports.engine.data.JRBeanCollectionDataSource($F{params})]]></expression>
</parameter>
Regards,
Lucian