Warp resistance of joined 1x lumber
You are always smart to join two narrower boards to achiever a shelf width rather than purchase the full width in one piece.  The joined boards are far less inclined to warp.  It is necessary to joint the matching edges to get a tight joint.  Clamp from both sides. This helps eliminate a bow in the final piece. I personally like dowels.  And if you use clamping dogs to keep the joints even biscuits are not actually necessary.  Our modern glues are more than strong enough alone.