"exactly during" is correct and usable in written English.
You can use it when you want to emphasize that something is happening at a specific time, that no time before or after this time is relevant. For example: "I will be able to help you exactly during my lunch break."
Why else would a devastating earthquake occur exactly during mass time on All Saints Day?
It's not that the group is bad exactly -- during the post-Strokes frenzy, many less-accomplished bands were championed by people who should have known better.
These signals on the Northern railways appeared exactly during the main phases of the strongest part of the geomagnetic storms characterized by high geomagnetic indices Dst and Kp (Ap).
In this way, the PVA/MMT hydrogel was prepared by immersing the frozen PVA/MMT sol into a borax solution, inducing crosslinking between diols of PVA and borate exactly during the ice melt.
The tyrannical character of Abraham Lincoln, the Rooseveltian perfidy vis-à-vis Pearl Harbor, the loathsomeness of the income tax, together with the oppressively imperial nature of modern America -- those were Wilson's ideas exactly, during his later years.
In response to Qassim's claims, MSF said its representatives were "currently engaging with the Libyan authorities in order to clarify what happened exactly during the incident and to ensure that similar events, that can put people in physical danger, do not occur in the future".
The latter regions were exposed to the risk of overinflation during the filling procedure, exactly as during tidal inflation in patients on mechanical ventilation.
I'll be exactly 26½ during the London Olympics.
Rottenberg wasn't exactly idle during that 57-year gestation period.
In contrast, eukaryotes duplicate their DNA exactly once during a discrete period between cell divisions.
"Hard Rain," Bob Dylan (1976) By my calculation, Bob Dylan appeared on TV exactly twice during the bulk of the nineteen-seventies.
