The Donaldsons were arrested in their County Down home and charged on 28 March 2024.
He resigned as DUP leader the next day and did not contest his Lagan Valley seat in the general election four months later.
He is accused of rape, four counts of gross indecency and 13 counts of indecent assault.
The offences allegedly occurred between 1985 and 2008.
There are two alleged victims.
Lady Donaldson will face a trial of the facts after being found unfit for a conventional trial under the Mental Health (Northern Ireland) Order.
Under a trial of the facts, she will not participate in proceedings. She was not in court for the trial’s opening day on Tuesday.