It sounds like they actually have a conventional supercharger in addition to 2 turbochargers. It'll probably be tricky to keep it all under control, but it could result in impressive performance.
The article is kind of confusing. At one point, they say:
"The 2016 XC90 (on sale early next year) is the first of this new generation of Volvos. It’s powered by a 2.0-liter four-cylinder engine with a turbocharger and a supercharger. (While the turbo works best when the engine’s already going, a supercharger forces in air with power generated by the engine itself, so increased boost is available from idle.) The supercharger runs from idle to 3,500 RPM. Above that, an integrated clutch disengages the supercharger, allowing the turbocharger to take over."
Based ont his paragraph, it sounds like a traditional supercharger that works up to 3,500 RPM. Later on though, we get this bit:
"We’ve seen plenty of twin-turbos before. Volvo’s innovation is adding a third—third!—turbo. But this one doesn’t charge the engine. It charges the other turbos. It’s electric, hence the name “e-booster,” and sends air into the two conventional turbos to improve their performance below 3,300 RPM, filling out the torque curve and eliminating that pesky lag problem."
Now we have an e-booster referred to as a third turbo, which runs up to 3,300 RPM.
So, the confusion is warranted. The article isn't entirely clear on whether these are two engine options, or a single engine with inconsistent details.