Warhammer 40k - Tau detachment benefit clarification
With the errata* released in July of 2016, this is now a much clearer rule.  The rule now reads that the shots are resolved at the same time, but does not indicate that the shooters are all counted as the same unit.  Just like the rule read before, the benefit of the improved BS and shared markerlights still applies, but you no longer get to share unit-wide special rules.  Darkstrider no longer confers the toughness reduction to all the shots in the coordinated firepower attack.  A Commander with the Signature Systems that buff his unit if he does not attack no longer confers ignores cover and twin-linked to an entire coordinated firepower volley.

*GW publishes errata and FAQs on facebook first for comments, then pushes them to their site later.  Bell of lost souls reposted them for those without facebook accounts here