When both sticks are in the top 'inside' corner, you mean full throttle/right rudder and full down elevator/left aileron right? That is not positions for an inverted flat spin. With inverted spin you give same direction aileron as rudder to flatten it out, with upright flat spin it is opposite aileron to rudder.
Some fun fly planes, like Pinwheel, will stay in the flat spin no matter where your right stick is, as long as you have full rudder and some power.
DKjens