Merging two individuals, or two copies of a dividual together to make a single mentality with two sets of memories
While virtuals can be copied almost trivially if there is enough computing power and space, merging two versions of the same virtual person is hard and quite expensive. As two copies diverge, their mental structure become more different and there is no longer a one-to-one correspondence between their minds. Just averaging them together or overlaying one neural net on the others does not create a viable person. The problem is even more acute when trying to merge two individuals who are not copies of one another.
The only successful way of merging two personalities is to allow a higher order intelligence such as an AI to examine them closely, to the extent that it can be said to intuitively understand both completely, and then compose a merged version. This of course requires finding a trustworthy AI at least one or two singularity levels above the virtual, which makes the operation expensive. Cyberia AIs on Fata Morgana, and a number of Keterist, TRHN and NoCoZo mindplexes have long specialised in this kind of work.
Somewhat cheaper but more risky methods of merging are available on many worlds; specialised AI systems sort through the separate personalities and memory-complexes and tag them with distinctive markers, which allows the merged personality to quickly establish which memories, and which opinions, belong to each original. After co-existing within a single mind for some time, the two personalities will (in most cases) integrate in one way or another. However in some cases the resultant individual may become disturbed or psychotic, requiring post-merging therapy.