-
A circuit wired single 2.5 twin and earth clipped in open air will have a fuse lower than 16amps is a rough rule of thumb. That is the cable will not catch fire with a 16amp draw. The cable is always the part that needs protection. The sockets are rated for 13amp draw, on a ring circuit that should include both sockets operating at 13amps otherwise it couldn't be 13amp rated.
2 3000w appliances running full pelt is pretty unusual these days.
It means you could put another double adapter in the second socket. It's convenience and smaller form factor.