XC90 T6 Lambda sensor question

[QBee]

I have been told by a diagnostics expert I trust that my daughter's 2005 XC90 T6 needs a new lambda sensor, the one before the cat.
Fine, decided to get one off one of the reputable spares vendors on the bay of the mighty flea.
Tried to get a part number, to be told by a Volvo dealer that it has two pre-cat lambdas, left and right.
But it's a transverse straight six engine, not a V6 (or V8). So why two? I can only see one pre, one post-cat lambda when I look down the back of the engine.

Does anyone out there have the knowledge to tell me which part I should be ordering please? And where the plug connects into the loom?

Thanks