the footbrake has an engine brake in the fly by wire accelerator.
I think you are slightly confused.

If you are in any of the "gear engaged" modes, ie, any position except park or neutral - when you have you foot on the brake, the DSG ECU takes a signal from the brake pedal switch and fully disengages the clutches. When you release the footbrake fully (even if you still have the handbrake applied), and the car is stationary, the DSG ECU instructs the appropriate clutch to engage to the bite point. As the road speed of the car matches the "theoretical" speed of the relevent gear ratio selected, the DSG ECU progressively applies full clamping force to the relevent clutch pack, thus transmitting full drive, without any clutch slip.
The footbrake certainly does not in any way "brake" the engine, because even with the footbrake applied, you can still rev the engine.
you can not drive a dsg even in confined spaces using 2 feet at once.
You certainly can use brake and accelerator together, but you need to know how to "defeat" the override. This is the same on all "fly-by-wire" accelerator pedals, be they DSG, slushbox, or manual.