The issue here is that you have two separate channels on the transmitter (left-right/aileron and forward-back/elevator) that you need to act as two combined channels for controlling your drive motors. As Coboxite said, the two ways to do this are to mix the signals at the transmitter end or to mix the signals at the receiver end. For the transmitter end I think that most people have transmitters that are intended for planes or planes/helicopters, but it seems that you have a transmitter that is intended solely for helicopters. I have much less experience with setting up controls for that type of control scheme, but I think that it may be do-able if more difficult than with a transmitter meant for planes.
For plane transmitters, elevon mixing is convenient because pushing one direction on the stick can give you an equal output on on two channels. For a helicopter transmitter (and anyone that has actually tried this can feel free to jump in and correct me), I think that you would want to set your swashplate type to 3-S (this being a type of mixing for helicopter inputs), and then I think that you would want to change the AFR to set the aileron and elevator channels to +/- 100%. I think that this could give you something that is functionally similar to elevon mixing, but I could be entirely wrong.
If it does seem to work at least as far as motor direction goes, (assuming you have fingertech tinyESCs to go with the silver sparks) you can run the calibration on each ESC so that the maximum input it sees from each direction on the stick gets translated to maximum output for the wheels.
If it turns out that this idea doesn't work, I think that your options would be either to get a different transmitter that allows you to do elevon mixing or to add a receiver-side attachment to do the mixing for you as Coboxite said. If you're sure that you're getting into this hobby for the long-term it might make more sense to invest in a transmitter with elevon mixing, otherwise it might mean having to add a standalone mixer into every bot you build/drive in the future.