Q: FaceTime only half duplex

Are you both using earbuds?  When using internal speaker and mic, Facetime uses speakerphone logic where the speaker is cut off when mic senses input. This avoids infinite loop feedback. Earbuds prevent the feedback, so you should get full duplex sound.

To anyone having full-duplex problems on their iPhone/iPad, before trying to change a bunch of settings or rebooting your phone, try removing your iPhone cases at each end of the call and check if that resolves the problem. It worked like a charm for me, the flip-up leather case my GF was using was blocking part of the microphone and thus creating the problem. Hope it works for you too.