Certified GKH Gardening Expert
Your plants probably shouldn't be next to each other. The Camelia prefers acidic soil in a shady location, whereas the Choisya also likes acidic soil but in a sunnier location. Camelias partner well with rhododendrons, ferns, and hostas. Camelias also need more water than the drought tolerant Choisya.
The Choisya may be suffering its location. For the Camelia, I researched yellowing leaves and three options came up: root rot, yellow mottle leaf virus or scale. Look closely at your Camelia stems for small, hard shelled scale insects. Also check the leaves for a mottled look, which may indicate the virus. These articles should help: