Consider that the speed control is getting 14.4 to 16.8 volts from two batteries in series. You can acheive a single battery setup by connecting the battery to one side but you need to jumper the other battery connection so that the circuit is completed. I would recommend that you just run two 7.4 batteries if you're running Lipo.
Both motors are wired in parallel which explains why the Titans fail within a couple runs normally. They are seeing 14.4 to 16.8 volts and they are really only rated to 12 volts max by the manufacturer. The 775 has two sets of leads to get the higher amperage to the motor but it's actually rated for 18 volts by the motor manufacturer.