You can use a Y-cinch cable. Whether it's this undersat subwoofer or another one, the principle is the same. You have to get the cinch outputs from the car radio to both subwoofers. Actually, one is enough. What else can you get out of a second one?

Unfortunately, we have never been able to try it out. We have therefore forwarded the question to our community so that someone who knows this device better can answer it.