The double air system is nothing new, and used to be common. Westide Lumber used it if I recall because the Swayne equiment may have had it, Feather River Railway (sg) also had it (I've run the #3, now Cass #11), and DMIR still uses it as far as I know. ATSF might have used it on the York Canyon coal train. It was a common system, but was on dedicated equipment. I believe Westside used a double check to the brake cylinders, but other more modern systems have been described as "automatic retainers" suggesting a different arrangement.
The proper use of retainers will accomplish the same thing, but the straight air system does offer some advantages in exchange for the added equipment and maintenance. AB and later brakes do have the advantage of the second or emergency reservoir, this volume being used to help recharge the auxiliary reservoir during a release and being added to the brake cylinder during an emergency. The "quick action" on WABCO and NYAB triples does not provide a true emergency. The WABCO ones dump the brake pipe into the brake cylinder sometimes adding 1-2 pounds of additional pressure and causing a local reduction, the NYAB simply vent the brake pipe to atmosphere during quick action.
Amtrak and most mainline passenger operators use a second pipe through the train that is for main reservoir trainline. In cab-car operations it is requird to provide main reservoir air to the cab car for controlling the brakes. Otherwise, when used, it charges the auxiliary reservoirs directly instead of from the brake pipe through the control valve (the modern "triple valve"). These systems are usually No. 26 equipment which use a control or reference pressure to compare with the brake pipe pressure to determine if a brake pipe reduction is in effect. It is also capable of recharging and reacting far more quickly than any AB series of brake.