TLDR; the front side is 23% efficient, and the rear side 20% efficient.

They don't actually give an overall efficiency but it implies a total of 43%. They compare this to typical panels also at 23% efficient, so it's really remarkable if true. Other emerging solar tech is up to about 32% but if that could also benefit from multiple layers then total efficiency could become insane.

Seems a little too good to be true, really, but great if so.

Edit: Yeah, I don't think these efficiencies can be added like that. I guess the overall efficiency will depend on how reflective the ground under the panels is, and they will extract 20% of that. Maybe that's why they don't give an overall rating.