Unfortunately no. Quarq does not offer a mountain bike power meter spider that can be mounted to the Cannondale SiSL 2 (or SI) mountain bike crank.
The SRAM XX1 Eagle power meter uses a proprietary crank to spider interface, and can only be installed on the Quarq XX1 crank arms.
To use a Quarq power meter you would need to purchase a SRAM XX1 Eagle Power Meter chassis (power meter and crankarms). The Quarq XX1 would then take the place of the Cannondale SiSL 2 mountain crank.
To see the full specs of the XX1 power meter, visit the Quarq website here, or visit your local SRAM dealer for more information.