So, I was wondering if more then one Mei are using the freeze attack on a target, does it freeze faster?
Answer
No
During Closed Beta, multiple Meis using their primary fire would freeze a target faster, but the Nov 18 patch removed this ability.
From http://us.battle.net/forums/en/overwatch/topic/19892439723:
HERO BALANCE CHANGES:
Mei
If multiple Meis are attacking the same target, their freeze effects will no longer stack
Attribution
Source : Link , Question Author : Shaeldon , Answer Author : Pikmario