Freezing with multiple Mei's

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

If you freeze someone with multiple Mei's it goes faster.

Source:

Turns out this part is no longer relevant.