I have never tried this but.........it makes me a bit nervous. Triple front derailleurs work because the zigs and zags of the inner cage plate engage the chain at different "center" distances depending on how high the chain is in the derailleur. When a triple is setup to use as a double, the lowest zig is never engaged because of the height of the inner chainwheel. On a full 34/50 compact, it might be that the inner chainwheel is small enough to let the chain start into that lower zig as if there were a granny there........but then as the chain is pushed out to the larger chainwheel, it must both move onto the middle zag of the derailleur and then up on to the top zig. All of those zigs and zags are potential trouble spots.
I'm not going to say it won't work but since normal double derailleurs sometimes have problems with certain compact cranks this seems to be asking for trouble. If it works, let us know.
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