Yes you need at least one shared relay. In the example, Alice and Bob share Relay 2. I didn't draw the lines, but Chad could share Relay 2 as well, he still wouldn't be able to see Alice unless he follows her npub explicitly.

Bob and Chad have 2 shared relays, but Bob writes to R3 and reads Chad's posts from R4. Chad writes to R4 and reads Bob's posts from R3.