Daisy chain another subwoofer

I have a rythmik f-12 subwoofer that I would like to add another sub to. I'm currently using the line level inputs. Can I use the line level outputs from this subwoofer to connect another subwoofer off this one? Thanks!
Yes. You can also connect one to the L channel, the other to R. They're the same signal, won't make any difference SQ-wise, so do whichever is easier.
Only thing I can think of that might be a problem is if one of the subs plugs into another circuit. They must all plug into the same circuit or you risk ground loop hum. Other than that you should be fine, and the more subs the better.
Are you sure this is what you're hoping for (stereo mode)?
Watch this video for an explanation of stereo vs dual mono setup.
Both methods work, but you might have a preference for dual mono setup. I suspect you would need a pair of passive splitters since you've asked about daisy chaining from one sub to the other. Basically, one splitter for each channel (1 in 2 out). Left side goes to left input channel on each sub, and same thing for the right side splitter to subs. Technical issues aside, I'm just letting you know there's another choice, and my feeling is most people choose dual mono for the reasons mentioned in the video link.
     The guy in the video is generally correct and ultimately comes to the right conclusion, all bass below about 80 Hz is perceived by humans as mono.  This  means we cannot localize bass tones below 80 Hz, in other words, we cannot perceive exactly where these tones or sounds below 80 Hz are originating from in a room or within a stereo sound stage illusion.
     While it’s true we cannot localize the fundamental bass tone frequencies below 80 Hz reproduced by the subs, we can localize the harmonics or overtones of these fundamental bass tones that typically extend above 80 Hz that are reproduced by our main stereo speakers.       Fortunately, our brains are able to associate these harmonics or overtone frequencies that we can localize with the fundamental bass frequencies that we cannot localize.  Due to this cognitive process, our brains are able to localize the fundamental frequencies and create the perception of exactly where in the room, and within the stereo sound stage illusion,  the bass tones below 80 Hz are originating from.       This whole process allows us, for example, to perceive the double bass as being located at the front left of the sound stage illusion and to perceive the drums as located at the rear center of the sound stage.
     Given the above and the fact that recording engineers have been routinely summing all recorded left and right channel bass below 100 Hz to mono on virtually all music and HT content on all formats for decades, it makes little sense to setup and operate all subs in one’s systems to anything other than mono.