For stereo connections, you don't halve power amp impedance - "it is what it is". So if the specs say each output channel is intended to drive 16 ohm loads, then thats what you should connect to it. (As a brief aside some amps have rear panel switches that can be set to useful impedances to cover various cabinet specs). Keep in mind that a 2x12 cabinet isn't stereo since it has 2 speakers - it is a mono 2 speaker cabinet that (as you correctly say) can either be wired up as a 16 ohm of 4 ohm cabinet using two 8 ohm speakers. So if your amp is most efficient driving 16 ohm loads, then wire up the speakers in series and have a good 16 ohm match.