RCA Stereo to Phono? (connecting B&W PV1)

I am contemplating the B&W PV1 subwoofer for a stereo setup consisting of a HTPC, external DAC and Bang&Olufsen Beolab 3 (active loudspeakers). The problem is that I downloaded the subwoofers manual and saw it has a phono input for line level and not stereo. I can only feed it RCA stereo.

I know there are "RCA Male to (2) RCA Female" Y-adapters that are used to feed a single output source signal into multiple inputs (I'll be using 2 of those to feed the DAC's stereo output into both the beolabs and the subwoofer). My question is can I use an "RCA Male to (2) RCA Female" Y-adapter to combine the audio information of 2 channels into one so that it goes into the subwoofer's phono input or is this impossible?


Any help would be greatly appreciated.