Could one make a Devil Fruit eat another one? 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.