What happens when you double the channel width (for example, use channel bonding) in a...

Doubling the channel width increases the noise floor because the receiver is now listening over a wider range of frequencies, which can include more interference sources. This results in a decrease in SNR (Signal-to-Noise Ratio), which can negatively impact performance, especially in high-density deployments.

From CWDP-305:

“Channel bonding increases the bandwidth but also increases the amount of noise received. The increased noise can reduce the effective SNR and result in performance degradation, particularly in congested environments.”

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on ProtocolandSpectrum Analysis