Q. Must I disclose to my client that another attorney will be doing some legal research & writing in the case?
The short answer is yes. A sample disclosure agreement can be found here.
However, many attorneys already include language in their representation agreements that indicates that outside counsel, expert witnesses, and so on may be consulted, in which case a separate disclosure agreement may be unnecessary.
In either case, few clients will object when they are told that a well-qualified, experienced, and cost-effective legal research & writing attorney will be providing the thorough, cite-checked backup they will need to support their arguments before the court.