yes it can be done, and the flue will handle the volume of exhaust gasses produced being a 12/12. bear in mind code does not like that installation and the code nazi's may be along shortly to tell you not to do it. however, once again the main concern when codes were written preventing 2 appliances in the same flue different levels burning solid fuel; was directed primarily at wood burning. if a chimney fire should occur from creosote buildup, the chimney would not easily be able to shut down combustion air. the second concern was that should you have a chimney with a poor draft, gasses could be produced by one stove, and exit into the house through the higher one. this is very unlikely however, and as long as you have decent stack height above the second appliance, and/or an interior chimney it will basically never happen. i would make sure to have a decent co detector in the vicinity of the second (higher) stove, but i wouldn't worry much about it, and if i were in your situation, i would probably do it in a heartbeat.
btw, if you're going through a combustable wall to acess the chimney make sure that combustables are not even close to the wall pass-through. at least 18" filled w/ fiberglass insulation. you don't want a fire to start behind a wall.