create two different global variables for e.g, vara, varb and pass vara in one sub report, pass varb in second sub report. keep on adding the dates to these variables with a speratior.
for e.g in sub report 1
vara := vara&totext(date)&";"
subreport 2
varb := varb&totext(date)&";"
now in the report footer of the main report u can store all the dates in two different arrays from the two variables and compare the dates.
The process is bit tricky, but i m sure this may work out for you.