Here’s how I’d do it. Sorry if this is a bit obvious.
1. Create a new report, with a new dataset defined by, for example,
SELECT fieldA FROM table1 WHERE dateField1 = @startDate
2. This should automatically create a new report parameter called @startDate with the same type as the dateField1 column.
3. Build the report (minus the subreport)
4. Create a second report (to be your subreport) with a new dataset defined by, for example,
SELECT fieldB FROM table2 WHERE dateField2 = @startDate
5. This similarly creates a report parameter @startDate with the same type as the dateField2 column
6. Check that the @startDate parameters in both reports have the same type e.g. datetime
7. Then add a subreport to the first report, and in the subreport properties, define the subreport parameter @startDate = Parameters!@startDate