How did they want the GA to phrase the request for volunteers to change seats? A couple would like to sit together, so would someone like to take a middle seat? or There are two deaf adults on this flight and because they're deaf, they need to sit together, so who wants to take their middle seat?

I would guess that they would not have wanted the GA to announce that they're deaf to everyone on the flight, nor would they view deafness as something that requires them to sit together, with the implication that deaf people can't travel alone.

I could see an argument for special treatment in the GA or FA trying hard to get them seats together if it's the case that one of them can do lip reading or can speak but the other can communicate only in sign language or by reading and writing, so that one of them could help the other to communicate with the FA on board or help to communicate in an emergency. This would be similar to the case of a traveling companion who provides assistance for passengers with other types of disabilities, such as feeding someone who is physically incapable of eating without help. However, normally if this is the situation, the passenger needing assistance would have their medical condition noted on the record and they would be approved for travel only if accompanied by a traveling companion who can provide the needed assistance. Then the person would never again be permitted to travel alone unless they could prove that the assistance is no longer needed.