It’s all about who nominated whom. Both members of the couple would have to have been nominated by a third party.
A co-owner can’t nominate themselves – they have to be nominated by the other co-owner or another owner, provided neither they nor the third-party nominee is standing for election.
So who nominated the first co-owner? If the nomination was invalid (and they are supposed to be done in writing) the wrongly nominated person should step down.
I have to add, however, that they could then be co-opted back on to the committee, by a majority vote of the remaining committee members, to fill the vacancy created by their resignation.
The question then becomes, how much trouble do you want to cause, bearing in mind that any decisions made by the committee are still valid until the discrepancy is reported?
This is who the Act says can be elected:
… an individual who is a co-owner of a lot … if the person is
nominated for election by an owner who is not a co-owner of the lot or by a co-owner of the lot who is not a candidate for election as a member.