We saw deers, guns and swords that have Devil Fruit powers. Would it be possible (with Vegapunks help) for a DF to eat a DF?
Would it explode like if someone ate two DFs? If not, one could try to eat that DF to get both powers (And should explode).
If it is possible, it is worth thinking about whether or not the resulting DF has it's own powers too. E.g. take a fox Zoan, feat it to the fire fruit (no offence to neither Ace nor Sabo here) and create a "Firefox".
Well, since both Vegapunk and Ceasar are doing weird experiments on DFs, i think that could happen one day and we can be sure they are working on it.
However, we can't know if they will successfully fuse two DFs as it will be decided in the future by the scenario. Only Oda knows.
Hope that helped.
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.
I could imagine that this is how blackbeard got whitebeards power. He lets his devil fruits power suck up the devilfruit created by whitebeards death so he inderectly has controll over both dfs
