It's official now. The announcement that Don Muang will re-open and that Bangkok will henceforth have two international airports was made yesterday.
Now, this could all be part of the face-saving operation allowing them to close Suvarnabhumi for good later because the "restructuring there is proving bigger than expected".
Be that as it may, assuming the new airport remains open there will be big problems in transfering passengers between the two. The Skytrain will not be ready for a while and, in any case, it doesn't serve Don Muang. There is, however, a railway line that runs out to the new airport and another that runs past Don Muang. It shouldn't be impossible to link the two, buy some new rolloing stock, and create a quick and relatively inexpensive express link between the two.
The only other thought I had was departures from 21 at Don Muang conflicting with arrivals to 19 at Suvarnabhumi. I hope the ATC guys devote some serious though to that. At present, all departures from Don Muang (it's always remained active) seem to be turning right.