Therefore, there will not be a functioning market for both versions; it would only cater for those clever hackers. Malicious hackers may be able to exploit the confusion of some ordinary clients to do double spends; but payment processors and exchanges will almost certainly deal with just one "bitcoin", the one with mining majority.
instantly the moment of the fork?
across all vendors across the entire world?
what if they are sleeping?
[/quote]
The XT fork will not be a "stealth fork" (like the BIP66 caper).
Again: if and when the blockchain tags indicate that 75% of the miners are ready, there will be a 2 week period during which everyone running Core (or other incompatible software) will be warned of the event and urged to switch to XT (or any compatible software). If and when that happen, you may be sure that all sensible exchanges and services will upgrade, together with the sensible miners among the other 25%. Perhaps even Luke. Perhaps even Mircea.
So, OK, there may be a few "rebel" exchanges left to cater for some thoroughly confused clients who did not upgrade, and the few "rebel" clients who refuse to consider bigger blocks and are willing to put up with 1 block every 2 hours... But I still don't see any great opportunity there.
(After the 2 weeks period, there will still exist only one branch, and hence only one altcoin, until someone mines a block larger than 1 MB. That may not take too long. The
average block size in early 2016 should still be 0.7 MB, but variable block spacings and empty blocks may create a backlog of more than 1 MB in the queue at some point.)