Monozygotic vs Dizygotic heteropaternal superfecundation
A normal singleton child is a result of the fertilization of an ovum (with 23 chromosomes) by a sperm (with 23 chromosomes). After fertilization, the cell (with 46 chromosomes) is called a zygote.  All normal human cells (other than ova and sperm) have 46 chromosomes.
Monozygotic twins (identical twins) occur when the set of cells resulting from the early divisions of a single zygote separate into two groups.  Each of these two groups of cells proceed to form a normal embryo, with resulting (if carried to term) in twins.  Since these twins came from the same zygote, they have the same 46 chromosomes and so are essentially genetically identical.
Now how could this monozygotic process occur with two fathers being involved?  Only by having two separate sperm combine with a single ovum.  This would result in the zygote having 69 (3x23) chromosomes.  This could happen, but would be an extreme case of aneuploidy (abnormal number of chromosomes). Such a genome is massively defective due to the presence of too many gene copies, and the embryo (whether singleton or twin) will be spontaneously aborted.  Only a few kinds of aneuploidy can result in a viable fetus, almost all having only a single extra chromosome (one of the smaller ones); the most common case is having 3 copies of chromosome #21, which results in Down Syndrome.
So, monozygotic heteropaternal superfecundation has not been observed because the resulting pregnancy is very unlikely to go to term. (It's just as unlikely when the two sperm are from the same man.)