Just a couple of points. Most seem to be assuming that an ammeter is being used to measure the current so only one wire from the battery is used. The OP is using a wattmeter, these are connected to both the ve and -ve wires out of the battery as they also measure voltage. Check the way you are connecting to the batteries as it seems that with one battery you are measuring current to both escs while with both batteries you are only measuring current to one esc.Also there is no need to charge the batteries separately as long as you have not run one down much lower than the other. As long as they are both near the same they can be charged in parallel without any problems, there are parallel charging boards available for this which can take 6 batteries in parallel for charging. As long as the batteries have the same capacity the fact that they are being used in parallel will mean that they stay at the same charge state.
Jim