So here is where the customer service rep comes out in me. The DXS needs to be programmed for elevon mixing using the Spektrum software. You must create a system mix that combines the two control surfaces. The other way to do it is to download a model like the Optera to the radio and then adjust the controls to how you need them.
Here is the link to the Optera file Optera Model File
This link is the google drive of every model you can download.
Now. I'll be honest with you. The DXS is the radio that makes me cringe when the customer tells me they are using it. Most of the time, it's because they are trying to use it for multiple models. Using a laptop and switching models every time you switch aircraft would be best.
The second problem I run into is non "techy" folks trying to use it. They don't understand firmware updates and how to switch models. The DXS works excellent when it's kept with the model it ships with, but after that, it becomes more trouble than it's worth. I know it's about $150 more, but the DX6e is an excellent starter radio that allows for multiple model memory and programming by computer or on the radio itself. If you look at the manual for the DX6e on page 31, it shows you can choose the model that already has the flying wing/delta mixing done for you. All you have to do is plug your servos into the right channel on the receiver.
As for your battery question, you need to buy or make a series connector. I have included a picture of one AMain sells as an example. The single plug is + and - however, the - off the one plug goes into the + of the other, and then the negative goes down to the main plug. Feel free to PM me if you have more questions.