Connecting two RC batteries in series is a common practice to increase the voltage output. This is achieved by connecting the positive terminal of one battery to the negative terminal of the other battery, and then connecting the remaining positive and negative terminals to the circuit. The voltage output of the two batteries will be additive, meaning that two 3.7v batteries connected in series will produce a voltage output of 7.4v.
There are several benefits to connecting RC batteries in series. First, it can increase the voltage output of the batteries, which can be necessary for powering high-power devices. Second, it can extend the runtime of the batteries, as the total capacity of the batteries will be increased. Third, it can help to balance the load on the batteries, as the current draw will be evenly distributed between the two batteries.