You could try syncing instructions in stead of objects. I use extrapolation (predict location based on active state) instead of interpolation (plot movement based on known points) to coordinate movement.
My way is much more complicated but for platform games it provides the results your looking for. What I do is call functions simultaneously on each device to sync the action instead of the object. Here is an example capx to demonstrate the theory. Open on multiple browsers, click connect. use arrow keys to move objects. Try it with 2,3,4 players and see
[attachment=0:16aa78t5][/attachment:16aa78t5]
Edit: I have reduced all the code from this example to simple plugin but haven't released the plugin yet (I'm debating on whether its worth the head ache of supporting it). In the mean time this example with out the plugin works if you want to give it try.