Coaches should be able to challenge until they lose one in the playoffs.
I think that the "two challenges max" rule is great for keeping things moving during the regular season, but this is the playoffs. Currently bad (compromised?) refs can just overwhelm the rule with bad calls. Mitch won TWO challenges in the first half and then had to sit there and watch them ignore hair pulling and holding on every play from then on out. If the refs keep getting it wrong then coaches should continue to be able to challenge until they lose one.
Gambling has brought new attention to the reffing in games and this change would both make losing a challenge more painful (and hopefully prevent ones that are just fishing for a different call) AND increase confidence that the outcomes of games are not being impacted by poor calls (purposeful or not).
This is the playoffs, they need to get it right!