Now I am not absolutely sure about this, but eating two Devil Fruits by a normal person leads to the person's death. Natural Devil Fruits in my opinion may not be able to do what you said. Since the way they get created is as follows: [taken from the One Piece wiki]
When a Devil Fruit user dies, their ability is reborn into another Fruit of
the same kind. Instead of growing from a plant, the ability simply
regenerates inside another existing Fruit, as seen when Smiley "died" and
the Sara Sara no Mi, Model: Axolotl transplanted itself into a nearby apple.
However, as demonstrated by Charlotte Linlin, if a Devil Fruit user was
literally eaten alive then the eater would straightforwardly gain the
ability. Another characteristic of a Devil Fruit's growth is that the stem
becomes noticeably curlier than it was when the Fruit was not endowed with
the ability. In the anime, it was also shown to change its color.
What we could now possibly look forward to is the case of Teach, "Kurohige", since he is the only person alive to have two Devil Fruits. When he dies it would be interesting to see whether the powers he has go into one Fruit or two separate Fruits.
Also the other cases wherein this could be possible is by Vegapunk's experiments since he successfully manufactured an artificial Fruit, maybe he could go on to create one which eats other Fruits, so it would be a mixture of artificial-natural Devil Fruit.
Ultimately, it does lie in Oda's hands and the answer can only be speculated on.