Former Rehab board member Frank Flannery and former Rehab chief executive Angela Kerins have written to the Public Accounts Committee this evening.
Ms Kerins has complained to the Committee on Procedure and Privileges concerning the way she was treated at her hearing.
Mr Flannery raised legal issues on the remit of the PAC.
The committee is due to discuss the issue tomorrow and whether moves should be made to seek to compel their attendance.
Mr Flannery's solicitors told the PAC he will neither accept nor decline the invitation to appear before the PAC until he gets a formal legal invitation outlining the issues to be addressed.
Ms Kerins's solicitors said she is still recovering and is not well enough to attend as a witness at this time.
They said this should not be construed as a refusal to attend when she is well enough.