obabel doesn't split molecules
Not knowing which version you are running, it is difficult to actually help you.

I am using Open Babel 2.4.1, and had absolutely no problem separating the two ethylene fragments from the following file (super.xyz):

12
symmetry c1
C       -0.498388809     -1.662363855     -2.140955132
C       -0.498388809     -1.662363855     -0.810797132
H       -0.498388809     -0.739022855     -2.713905132
H       -0.498388809     -2.585704855     -2.713905132
H       -0.498388809     -0.739022855     -0.237847132
H       -0.498388809     -2.585704855     -0.237847132
C        1.730729269     -0.175065928     -2.629761741
C        1.730729269     -0.175065928     -1.299603741
H        1.730729269      0.748275072     -3.202711741
H        1.730729269     -1.098406928     -3.202711741
H        1.730729269      0.748275072     -0.726653741
H        1.730729269     -1.098406928     -0.726653741


Using the command

$ obabel super.xyz -Ofrag.xyz --separate -m


results in frag1.xyz and frag2.xyz containing one ethylene each.