Would need to know more about what kind of report it was and what sort of data you were using, but my initial thoughts would be that you could move the data in Detail a into the Report Header, which would mean it would only appear once at the top of the report. You could then hide Detaila, or just delete what is in Detaila and move the contents of Detailb into Detaila, then delete the detailb section.
But as I said more info may be helpful, explaining why there are two details sections.
"Stupid isn't not knowing the answer, it's not asking the question