Paralleling batteries is Bad Bad Bad.

Its OK to parallel 2 batteries for the brief moment it takes to turn
on battery 2, just before turning off battery 1 (or tail battery on/
off) This way you don't interrupt current flow to your flight
recorder. I have used this method for 30 years. Please don't tell me
it will cause a voltage spike. I have tested it by hooking a dead
battery directly to a freshly charged one...........................no
spike, didn't blow a 3A fuse all that happened was 220 m/a current
flow as the good battery tried to charge the dead battery. While we're
speaking of voltage spikes, you can get one coming out of your radio
when the power is interupted and this can cause trouble with other
units in the system ....................see Ilec manual warning about
this.
JJ