Sheldon Brown's Hetchins had a 50-28 TA double: http://sheldonbrown.org/hetchins/index.html , so it's been done. It's a helluva heave from the small to the large chainring, though.

FWIW: when you move the large chainring to the middle position, just replace the chainring bolts with those made for single chainring arrangements. They're shorter and will tighten the chainring to the arm just fine... maybe.

The outer ring is countersunk for the bolt, and putting it in the middle position may cause the chainring bolts to interfere with the chain when it's on the small chainring. Also, the chain may hit the large chainring when you're in a small chainring/small cog combination.

In other words, it's an experiment. It might work. It might not. Try it and report back.