Philip Bowen (NCE 24 February) states that a currach catamaran 'is capable of four knots in up to gale force 4 in the open sea'. These boats are light, flexible, and designed to ride the waves like a seabird. Rigidly lashing one each side of a four tonne stone will destroy the seaworthiness, and they will be swamped and sunk in the notoriously choppy waters of the Bristol Channel.
I doubt if the frames will stand the asymmetrical rowing forces for long, or if ten rowers will be able to move the contrivance to windward at all, even if they have Steve Redgrave on board, which they won't. Nick Price thinks 'if it doesn't work it's still great'. If it doesn't work it won't be great, because you'll have 12 people sitting next to a four tonne stone heading rapidly for the sea bottom.
If they are determined to proceed with this project, they should construct decked-in unmanned flotation units fixed to the stone, and attach two very long towropes to a couple of curraghs. They will need to have someone standing by on each boat with a sharp axe (Stone Age of course) in case things go suddenly pear-shaped.