Because if the patch im playing with say has any vibrato, tremolo, or even chorusing or other dsp, by doubling the octave you wouldn't necessarily get that extra punch at the desired frequencies because there'd be way more noise (noise as in many more frequencies besides the fundamental, not really all-frequencies-noise) because of the nature of timbre and that could muddy up the low end.
you'd be fine if you played them both at the exact same time. the vibratos should be syncronized