Double sockets are still only rated at 13A not 26A.
You can quite safely add another socket, just check the wiring in the one you want to extend. If there are two sets of wires then extend the ring to a new socket.
If there is only a single set of wires then it's a spur. You can extend a spur but you'll need to add a 13A fused connection unit before the socket you are spurring from.
A general rule of thumb is 'you can't take a spur from a spur'. There are some circumstances where this this doesn't apply but generally it requires that the cable to the socket is protected by a fuse.
Double sockets are still only rated at 13A not 26A.
You can quite safely add another socket, just check the wiring in the one you want to extend. If there are two sets of wires then extend the ring to a new socket.
If there is only a single set of wires then it's a spur. You can extend a spur but you'll need to add a 13A fused connection unit before the socket you are spurring from.