It is very common to hook two batteries together in series to obtain voltages that are otherwise not available. For instance, several EDF jets call for an 8-S LiPo, so many people connect two 4-S LiPo's in series to get 8-S voltages. Just be sure the batteries are the same and in good condition (make, type, size, age, C-rating, mah rating). This fits you situation exactly. You can buy "serial" cable connecter adapters to hook two batteries together in series. However, you will have to unhook the batteries and charge them separately in order to use the balance plugs. This should not be a problem since LiPo batteries should never be charged inside the airplane. Here is a link to a typical serial adapter using XT-60 connectors: https://www.buddyrc.com/products/ser...direct-connect