Hardy hibiscus or Rose of Sharon are usually good candidates for containers. however they do need regular and deep watering. Although you have treated each the same, I suspect that at one time the yellowing one actually dried out before you noticed it. The yellowing leaves will drop on their own and as long as it looks healthy otherwise, don't worry. You don't say if you are feeding these plants at all. I would add liquid fish emulsion at half strength every week or so to their watering regime until the end of the summer to build up the plants. Be sure to follow the instructions on the package.