Something like this?
Design the spark line as a sub report with the second data set in it and use a parameter for the metricname and it should work and a little where clause in the data set to take the parameter and work out the dates needed. (Image attached)
This should work. You really have no choice but to use a additional report (in this case, the above mentioned subreport) with its own data region as you can only attach a single dataset to any of the data regions.