I only bought the -5D on February 8th, and it is now 5000XP into Mastery, with the Master option being available.
So I would regard it as confirmed that you only need 1 at a time.
I'm not sure if I read it first or just tried myself. I previously referred to this thread:
http://mwomercs.com/...cies-explained/
But couldn't find any hint there
This may even be true if it's the first mech in a weight class. But I cannot tell because I always used (or still will for Assaults) the first chassis to unlock master in parallel.
One other thing I noticed since UI2.0came out: it seems like the Convert XP button isn't locked anymore before completing the basics. Not sure if this is intentional.
Edited by Arnie76, 01 March 2014 - 12:59 PM.