Between ANY two general points there could be a potential difference, hence a voltage, for whatever reason. If you accidentally charge yourself on a carpet, then there is a potential difference/voltage between you, and for example a radiator. This is why you sometimes get a little shock when touching a radiator. The shock indicates you that there were charges traveling between you and the radiator to balance the potential difference, and the voltage will be gone after you touched the radiator.
This means between any two batteries, negative or positive poles, could be a potential difference (depending on what happened to the batteries before that). However, notice: This potential difference will be gone as soon as something that allows currents to flow touches both batteries (for example if you take them in your hand). This means this potential difference between the batteries can NOT be maintained as the batteries can maintain their internal voltage between positive and negative poles to allow electric current.
Since there could be a potential difference between any two points, sometimes things get connected to each other or to the ground, so this voltage between them does not interfere with there functionality. For example if you have a stereo and a record player, you have a conducting wire that connects them. If you take this away, then the potential difference between the record player and the stereo will be amplified if you turn the stereo on and you will get an annoying hum.
If you have two USB-wires in the same supply, for example a PC, then everything should normally be at the same potential (meaning there is no unwanted voltage between them), since they have a ground wire, which could actually connect them to the ground through their connection to the PC, or at least to each other and to the PC, so there is no unwanted potential difference/voltage between the components.
