A Very tragic event… The Ultimate cause is that 1 of the 2 were at the wrong Altitude! 1 of several things happened in my opinion!
1.) Someone not paying attention
2.) Altimeter with Misconfigured Barometer settings
3.) Error in Altitude indication in the cockpits of one of the planes.
It will be interesting what the NTSB comes up with on this one… It is such a shame to lose these 2 crews that were so experienced and the plane that was such a part of great history.