I don’t know, off hand, how much of the line level input your subwoofer needs, but I would assume a splitter would reduce the output on each side by 50%. You might try, and let us know the results, along with the make and model of your subwoofers. 

Usually line out jacks have a low impedance and line in jacks have a high one so you can split with minimal loss of signal level.

Never seen Sonos specific numbers but other gear talks about 10,000 Ohm input hack impedance while outputs are around 100 to 600 Ohms.