Passing multiple parameter into MDX query from .NET application

hi,all

here my MDX query:

SELECT NON EMPTY { [Measures].[aaa], [Measures].[bbb], [Measures].[ccc], [Measures].[ddd] } ON COLUMNS, NON EMPTY { ([datetime].[datetimename].[datetimename].ALLMEMBERS ) } DIMENSION PROPERTIES MEMBER_CAPTION, MEMBER_UNIQUE_NAME ON ROWS
FROM ( SELECT ( STRTOSET(@[mydate], CONSTRAINED) ) ON COLUMNS FROM [cube1])
WHERE ( IIF( STRTOSET(@[mydate], CONSTRAINED).Count = 1, STRTOSET(@[mydate], CONSTRAINED), [datetime].[date].currentmember ) )
CELL PROPERTIES VALUE, BACK_COLOR, FORE_COLOR, FORMATTED_VALUE, FORMAT_STRING, FONT_NAME, FONT_SIZE, FONT_FLAGS

ReportViewer control is used in my asp.net application and i pass @[mydate] by:
p[0] = new Microsoft.Reporting.WebForms.ReportParameter("mydate", "[datetime].[date].&[2012-06-15T00:00:00]");
it is works!

however, i want to pass multiple parameters, like this
p[0] = new Microsoft.Reporting.WebForms.ReportParameter("mydate", "{[datetime].[date].&[2012-06-15T00:00:00],[datetime].[date].&[2012-06-16T00:00:00]}");
there is no result in my report.
why?