How to bowl a flipper-doosra
This should work for an off-spinner (or orthodox slow left armer) who has a high action (near vertical arm at delivery) and large hands.
For an off-spinner - the usual grip is between index and middle fingers across the seam , with the ball stabilized by the thumb and ring finger, also resting on the seam.
To deliver the flipper-doosra the grip is very similar, but with index and middle fingers just spread apart and resting on top of the seam - while the ball is actually being gripped between the thumb and ring finger, which are on opposite sides of the ball, thumb and ring finger on the seam.
For the normal off-break, at the moment of delivery the seam is angled towards leg-slip.The ball is spun by rotating the wrist and forearm to move the seam in a clockwise and pointing-forward direction.
For the flipper-doosra, at the moment of delivery the little finger side of the hand is presented to the batter, and the seam is angled towards first slip; and the ball is spun by snapping the ring finger down, so the ball pivots on the thumb - just the same motion as you could use to 'snap your fingers', using ring finger and thumb.
So, from the batter's perspective, there is only a small angular difference between the delivery angle of the off-break (stock delivery, coming in towards the right-hander) and the flipper doosra (moving away from the right-hander).
Given that the grip is so similar, I think the flipper-doosra would be hard for a batsman to 'pick'.
The disadvantage of the flipper-doosra (assuming that it could be practised until it was controlled) is that the finger snap is tiring for the fingers, so the delivery probably could not be used very frequently - and the ring finger is neither very powerful nor easy to control.
If the fingers actually snapped together, made a snapping noise, then this would potentially alert the batter that the variation which goes the other way was coming.
Otherwise it might be worth a try.
Note: The flipper-doosra could also be used to bowl an off-break - therefore making it a double-bluff against batters who manage to 'pick' the flipper action. This would simply be done by rotating the wrist before release, so the seam points towards leg-slip (instead of towards first slip).