This a more delicate decision than it appears.
If the game has 3D grafics as they show on those shoots, that means that the 2 screens use would drop framerate to 30 in each. And on a fast paced game that would be a bad move. As we do not know how is it's pace we can't decide for one or another mode.
Those are my points:
case a) The gameplay it's slowpaced and the speed at which elements are scrolling on the screen are slow, so the player can follow easily the development of the stage. - make it 2 screens by default, just to sustain the 2 screens cool factor.
case b) The gameplay is quick paced and in order the keep the hiperaction of the player enemies and their amo should scroll really at high speeds. - Make it one screen and gimmick on the second (a radar which no one will care like in Nanostray).
case c) There's hundred interesting posibilities if you can add to the gameplay constant perspective changes. Check classics like Axelay to know some tidbits ¡and imagine the posibilities! - If the developer got to that point then, of course, go for 30 fps 2 screens with slow paced gameplay. Also, remember you can swicht from 2 3d screens to one 3D screen, ofering the posibility to fast the action for some stages (again ¡check classics!)
Of course, Nibris can contact me for more in deep help