To split what?
A buttkicker, in order to operate correctly needs a sound device capable of 5.1 audio with 3.5mm output ports so you can feed the subwoofer output directly into the buttkickers amp. The Rifts USB based audio is only capable of 2 channel stereo sound, it has no option to add additional channels. Buttkickers can be setup to run off any of the sound cards 3.5mm sound ports, but by using any other than the sub you will introduce unwanted high end audio which you then have to filter out on the amp. Ideally you would just want to the games 5.1 bass track sent to the Buttkicker, and you loose this ability with any USB headset, Rift headphones included.
I see people using so called "Stereo Y Splitter cables" on these systems, in 99.9999% of cases these cables merely take a stereo 3.5mm output, and split the left and right channels to each of the "Y" ends. So left channel mono on the left split, right channel mono on the right split. Again not at all desirable.
You "could" virtually split, using Virtual Audio Cable or alike, the 5.1 feed so that your sound card still outputs to the sub but directs the remaining channels to the Rift's audio. However this introduces lag which is undesirable.
Hence why I would never intentionally purchase USB headphones, or bluetooth ones for that matter.