Freezing with multiple Mei's
Solution 1:
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
Solution 2:
If you freeze someone with multiple Mei's it goes faster.
Source:
Turns out this part is no longer relevant.