If you’ve got a dead cell in a battery, I’d be considering getting rid of the entire battery. Not just removing the dead cell. If one cell is bad, what is the condition of the others? But that’s me.
Personally, when I’m at home or if I’m not pressed for time in the field, I always just charge 1c. No matter the cell count, C rating or MAH. It’s easier on the battery and they seem to last longer for me. In the field I’ll 2c charge the smaller 4s and 3s batteries for my FPV gear but that’s only after letting the batteries cool after their last flight.
If you are using a parallel charger, make sure the cell voltages are close before you plug in the two batteries together. Personally, I like to see cell voltages within about 0.1 volt between the batteries before I connect them together. Again, just easier on the gear. If you have two 4200mah batteries connected to a parallel board then a 1c charge would be 8400mah. But as I said before, I recommend the 1c charge.