Electrical questions

1) Should aircraft use switches for both positive and negative (SPXX
vs. DPXX switches) or just the positive (especially in application
from point 2 below) ? I was planning on switching just the positive
only and have the negative side permanently connected; both batteries
and negative of each circuit joined together.
2) I am about to separate radio and transponder into circuit 1 and
rest of the instruments into circuit 2. I will have two batteries. I
am planning to use two switches; one for battery 1 and one for battery
2. Each of the switches should be able to switch a battery to either
circuit 1 or circuit 2. I would also have another switch that could
connect circuit 1 and circuit 2 (they need to be connected while
flipping the battery switches to ensure constant power supply). I
would also need a couple of diodes to make sure one battery does not
drain the other battery and that I don't get overload in any circuit.
The question is what diodes should I use?
3) Does anyone have another solution for battery switching maybe
simpler one?