Do  not utilize two of the "newer" D cells in parallel for increased current to power low  voltage filament circuits etc.  They often will discharge into each other creating an  imbalance and an interesting and dangerous "meltdown." I've discussed this briefly on one of my BC-611 pages and the link is below.  Never store these D cells in a parallel configuration inside a dwelling or vehicle.  This meltdown  has been experienced by several collectors.  After the first couple of meltdown events I wanted to blame Duracell but this melt down has also occurred with Energizer batteries. BTW in most cases a single D cell can supply enough current for most 1.5 filament circuits(BC-611, BC-1306,GRC-9)  so the parallel configuration is not needed.
k4che

http://k4che.com/BC-611%20Filament.htm