You can not plug them into a socket on a cooker connection unit as you can only plug one or the other in at a time (So that would be no use for cooking) You can not use a twin socket as the most current you can draw from a twin socket is 13A You would be better off getting an electrician to "hard wire" them both in for you.
In the main yes you could fit a double socket, however it depends on make and model of socket as to if you can draw 26A, 20A, or 13A, in the main 13A limit is only with special sockets, for example with a filter built in or RCD built in, most standard MK sockets allow 26A.
As to if good plugging in that's another question, any fuse gets hot, that's how they work, with an overload a bit of wire melts and circuit is broken, so the plugs need free air to cool, with a 13A oven often the plug is where the ovens cooling fans keep it cool.
However although continued use at 13A may be a problem as with an immersion heater where when switched on cold it can run for 3.5 hours before auto turning off, most hobs and ovens will start cycling on/off within half an hour so unlikely to over heat.
However there is always an exception, the range cookers can draw power for a very long time, so would not use a plug for a range cooker, neither would I use a plug where the socket is not in free air, or air circulated with a fan.
It is common sense, try it and after just see how hot the plug is, don't burn yourself be careful, it will get warm, but you should be able to grip it and pull it out, if too warm to do that stop using it.
My hob does work off a 13A plug, however only one heat area, domino hobs quite often are protected with 13A fuse, however 3 and 4 heat area hobs need some form of power sharing to work on 13A, with an induction hob since they use less power this is not normally a problem, but with halogen it may cause the plug to get very hot as it can draw 13A for an extended time.
However the whole idea of making hobs marketed as being able to run off 13A is that you can simply plug them in, so to simply say no you can't would be against manufacturers recommendations so could not say don't do it, but I would advise some caution.
First of all the original cooker will not be on a ring main, it will be on a radial. The reason I mention this is that the cable for the radial will be 6mm, In order to make a sub radial, you will have to loop from one point to the next. I have never seen anyone get two 6mm cables into the back of a standard socket. I would suggest in this instance you should call an electrician.
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