If there is current flowing through the contact, there is arcing potential. It is a well known cause of battery explosions when disconnecting batteries. Unless you have a zero-impedance battery bank, and zero-inductance battery connections, sparking will occur.
In practice, charge and discharge currents are large, battery connection paths are inductive, and battery impedance is significant at charge/discharge current levels.
In theory 'both connections' might be at the same voltage, and both sides of the connection might be at the same voltage, but in actual practice this is not effective, and disconnecting half a battery bank can cause sparking, and, for power applications, can actually cause persistent arcing.
Disconnecting half of a battery bank is significantly less dangerous than disconnecting all of a battery bank, or disconnecting the second half of a battery bank. But even when building a double-battery bank, proper connect/disconnect switching should be provided -- protected contacts or DC circuit breaker or switch gear -- so that the current can be zero'd or the arc suppressed.