It uses the GRE striping IP to send traffic for one radio down the other link (via port-channel IP load balancing).
On top of speed, you are also gaining redundancy. You can create your LACP groups across stacks and if a stack member dies, your APs will stay up.
For us, the benefit is the redundancy, not the bundled speed.
Note that currently there is no PoE redundancy. If the switch that is providing the power were to die or reload, the AP would reboot.