What happens if you connect two batteries of the same voltage positive pole to positive and negative to negative?

If the batteries have EXACTLY the same voltage, then nothing will happen, because there will be no voltage drop in the circuit, and hence no current. If the batteries have SLIGHTLY different voltages, then there will be a voltage drop in the circuit. Since there is low resistance, there might be high current, causing damage and/or an explosion. In all probability, however, there is enough internal resistance in the batteries to simply cause them to equalize in voltage.