One crude solution to increasing the "speed" of an image sequence (assuming it's all the same shot) by 2X is to:
1. Drop 3 of every 4 frames of the shot (i.e., take every fourth frame)
2. Use MFlowInter to generate intermediate frames to smooth out the motion
This would transform the shot from:
X X X X X X X X...
to
X I X I...
where X=original frame and I=intermediate frame.