I was watching the battle in
Generations on YouTube. More circumstantial evidence is presented.
The Klingons fire there weapons in bursts. Then they seem to stop firing for a moment. Or the 1701D is hit, but the ship is seen to shudder under the impact. This could be interpreted as the 1701D changing its shield frequency, and the Klingon weapons impacting the shields. The Duras sisters could also be waiting for their next opportunity to reset their weapons to the new frequency. Also if the 1701D's shields are being hit, it could provide a bit of cover for their eyes in engineering.
Then one of the Klingons tells the sisters that their shields are holding. This would seem to imply that the 1701D was firing its weapons. It's at this point that the Duras sisters decide to target the bridge, in essence trying to take out the bridge before someone caught on to what their advantage was.