The new convention for happy co-existence with the pared-down font engines packaged in devices like the iPhone is to use two 't' characters after all, but add a slanting accent underneath the first 't' (or whatever letter it is) to indicate that it should only be 'pronounced half'.

It's like Nastaliq's (the Urdu script) jazm character: http://users.skynet.be/hugocoolens/newurdu/specials.html

I think it's a sane move.