The use case is two people want to open a channel to each other, both want to contribute funds to the channel (in this case the same amount). Its a normal single lightning channel, but it starts out with liquidity on both sides and each side's contribution remains on their side of the channel. This is possible because both ends of the channel are "cooperatively opening" the channel, rather than one side being the sole initiator and funder of the channel.

Nothing big like that. Just regular plebs opening to each other.