You dive sidemount primarily for redundancy (and secondarily for coolness). But owning two tanks is not the same as being redundant. Success is demonstrated by how you handle a failure.
Sidemount exposes your sides. In a backmount rig, the tank and wing form a thermal barrier. In sidemount, your ribs are directly in contact with the water (and the cold tank). This leads to the most overlooked principle: Sidemount- Principles For Success
Be prepared to clip and unclip tanks mid-dive to negotiate tight restrictions. You dive sidemount primarily for redundancy (and secondarily