There have been $2 chips used as rack chips, particularly in Northern Nevada, on $2 minimum blackjack tables, back in the day.

As examples, do a query on ChipGuide for Nevada $2 denomination chips. The result shows 90+ chips, but includes some extraneous ones. About half are actual $2 chips.

See older chips for El Capitan, Harrah's, Harvey's, Soper's Montgomery Lodge, Regency-Laughlin, Sahara-LV, Sturgeon's and some others too. There's a couple of Pan chips for $2 condition games as well.

As I recall, in the days of $2 minimum BJ tables, the casino could operate a table with half the chips, using $2 chips vs. $1 chips. Also, as winning hands were paid in those $2 chips, the bettor was more likely to double-up to $4, creating a bit bigger handle exposed to the house's vig.

See query facility at: http://chipguide.themogh.org/cg_query.php