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A fully equipped, properly weighted tec diver may be as much as 20 to 50 pounds negatively buoyant at the start of a dive. Why? Because that's the weight of just the gas in the full cylinders.
Therefore, a tec diver needs at least two ways of controlling buoyancy a primary and a backup. Since you can add air to a drysuit, some individuals suggest that a double-bladder BCD essentially two BCDs in one is unnecessary if you tec dive in a drysuit.
While this may work in some situations, it doesn't stands up as a universal practice. Most drysuit manufacturers will tell you that their suits weren't designed for the stress of being filled enough to offset 20 or more pounds negative buoyancy. Even diving dry, then, you often need a double-bladder BCD. Unfortunately, some divers believe otherwise and they're often wrong.
If you think you only need a single-bladder BCD, find out. Do this: Kit up in all your tec gear with full cylinders and properly weighted (i.e., able to maintain a 15-foot decompression stop wearing near-empty cylinders). Go into a shallow, benign environment. Deflate your BCD and then attempt to ascend and maintain a deco stop at 15 feet by hovering for 15 minutes using only your drysuit to control buoyancy. Do some gas switches. Share gas with a partner. Go through all the primary and emergency procedures that apply to tec diving using only your drysuit to control buoyancy. Assume a head-up position to be sure most of the air doesn't simply escape around your neck (which would mean you can't be vertical in a backup buoyancy emergency).
Now ask yourself whether you could reasonably expect to handle a failed BCD using only your drysuit while task-loaded with decompression and the stress of failed equipment. Even if you could, what would be the more reasonable way to manage the emergency?
When you're done, you'll know whether you need a double-bladder BCD for open water, heavy-weight diving. Chances are, you will.
