I play in a live rock band with a drummer using a Roland VDrums drum kit which requires a solid bass response out front for the kick drum so we use a 2 JBL 15 inch subwoofers. One may be fine but we do use 2 of them. But if we didn't have an electronic drumkit I think one would be fine.

We do have the sub woofers separated on each side of the stage but that is more because they hold the poles for the 15" PA cabinets. I believe the correct technical answer is ( and you can verify this with a google search ) is that the position of sub woofers simply doesn't matter. Sub bass frequencies are NOT directional like higher frequencies and placement doesn't affect stereo positioning. Also related to this if you are using 2 sub woofers they provide better bass response when placed together rather than separated