Sorting grid with 2 data sources

Hello,

I have been searching this question for days and i could not find any answer. My question is:
I have 2 data sources in my form. And i have a grid. This data sources are joined to each other with inner join. I have fields of both Data Source on my grid. Now i want to sort this grid with following order:

ds1.Name
ds1.Surname
ds2.ExamDate

The grid Data Source is ds1.
How can i sort with this way?

Thanks a lot;