I just made a new modal interface for my car. It has normal mode, and hill mode. Toggle by clicking the mode button next to the start/stop button.
In hill mode, press the accelerator to slow, and the brake pedal to accelerate. There will be a light just behind the steering wheel to tell you what mode you are in. I recommend looking at the road while you drive.
This is how using 'shift+enter to linefeed', except when it is 'enter to linefeed' feels to me.
A less extreme metaphor: there are vi, emacs, and notepad people. Each gets used to their interface and it becomes physical memory. Very very few of them toggle back and forth on a paragraph to paragraph basis - that way lies madness.
In the mobile app, there is no issue using two separate keys to perform these independent functions regardless of input mode. Surely a more consistent customer feel would be achieved by giving an option to change this default functionality.