I don't have any direct service level evidence for the Astoria Line during this period. Both the BMT and IRT were each limited to a minimum headway of 4 1/2 minutes on the Flushing Line after express service started in 1939. The BMT had no choice but to run their shuttles onto the Astoria Line. If the IRT chose to duplicate the Flushing Line service levels, then the combined IRT/BMT service level would have been 27 tph. This should place an upper limit for the service level during the 1939-1949 period.
Both operators used shorter trains compared to today's 600' length. This reduced the time that it took to transverse the switch which is the physical factor that limits capacity at Ditmars Blvd. So, they should not have had that much trouble turning around 27 tph on two tracks.
They never used the center track on the Astoria Line for express service. This meant that the a disabled train could be moved onto the center track to keep both pockets clear at Ditmars.