I can understand 2 appliances with power consumptions over 2000 watts each but this boiling tap is rated lower at 1600 watts when heating which is for a few minutes here and there, not constantly for hours on end
Let's do a risk assessment (just concentrating on the power/overheating issue):
Is the wall your twin socket will be installed in made of brick, or other non-flammable material?
Will it be installed in a metal back box?
Will you use a twin socket from a reputable manufacturer? (which will be made of non-combustible material)
Can you use a torque driver when tightening up the socket terminals ?
If so, you are not going to set the house on fire, are you?
On the other hand, if your kitchen fitters apprentice mounts the socket in a Chinese knock-off plastic back box into a kitchen cabinet made of honeycomb cardboard, we might be more concerned!
I appreciate others have seen the occasional bad thing happen, but that shouldn't automatically invalidate the options.
It's not against regulations, it's all down to your judgement.