Since sound is waves, if theyre opposing theyll cancel each other out, and youll hear nothing.
Not likely to happen with the small wavelengths of high frequency satellites, but more of a problem with subs, which have generally large wavelengths.
Ive never heard of a sound card with 6.1 sound, AFAIK the most cards have is 6 outputs (5 for sats and 1 for sub).
If it really does have 6 outputs and no LFE, I suppose it would work, but ditch the extra 2 speakers, and put one sub on top of the other, that way the waves are less likely to oppose each other.
Honestly though, considering the range in which the speakers are likely to be placed, you really really dont need 6 channels.