You may want to consider reading the Smart-Fly and other info on redundant power supplies to a single receiver. There are potential issues with connecting two batteries to one receiver's servo buss, especially if the batteries are not matched or the charges are not the same (e.g. the batteries will equalize via the Rxer buss.) Also if one battery fails, there is no mechanism to isolate the bad battery from the Rxer or the other battery.

Lots of people do connect two batteries to the Rxer buss. However that does not mean it is correct or even will work as desired. It can even introduce new problems. If it is large plane with lots of digital servos, then I personally would never consider tying two batteries to a Rxer's servo buss. That is why Spektrum has powersafe and there are products out there like Smart-Fly's Battery Share for other brands that did not consider a method to address the issue.