Which codecs support dual-channel or 16 kHz recording under Hi-Fi voice?

GSM, G.726, and Truespeech support neither — both sides of the call are mixed into a single 8 kHz channel. PCMU and PCMA support 8 kHz only, but each side is recorded on separate channels. Among all non-Opus codecs, only L16 supports 16 kHz (via a rate parameter), though it currently defaults to 8 kHz since the rate is not passed.