[Breadbocks]

Some of my FRC team and I are using parts from previous competitions to build a robot for outreach. We already have so much stuff on it that when we put on a battery and run it, the voltage never goes above 12, never mind 12.4 or whatever it normally hovers around.
When we were coming up with solutions for this problem the only solution we could really think of was to run a second battery on it. If we were to do that, how would we do it? Would it be 2 completely independent circuits with power distribution boards and digital sidecars and stuff, or do we need to run them together into the same power board? If the latter, should the batteries be in parallel or series?
Thanks in advance for the help.