Dipygus is a fundamental flaw in a very early embryonic stage. It involves a split in the back bone, yielding two pelvises. The tail merge you require would involve merging the two inner legs, that would be one leg from each pelvis.. Maybe you've seen post-operation images, but at first, the child will have to separate lower bodies,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4155636/ (same case, also post-operation image)
Suppose the tail formation would succeed, there will be an opening at first. An aspect you'll have to decide about is procreation. Human patients have rudimentary sexual characteristics.
The inner legs need to be merged to form a tail, but apes don't do operations. Because of the Dipygus origin, the swimming tail would be relatively short at first, because Dipygus patients have underdeveloped inner legs. the shape of the body will become very wide and bulky. Water resistance as a result of this shape will not allow fast swimming. They will be vulnerable to predation.
There are some issues that also impact "classic" mermaids. For example, ape eyes were optimized by nature to support 3d binocular forward view. That cannot be undone. Your merape will need to maintain swimming belly down, looking down (=nowhere), or head up, maintaining a strong, flexible neck and shoulders, maybe putting the forearms to work for swimming.
They'll change beyond recognition
When successful (which I doubt, to be honest), in a few million years, your merapes won't be recognizable as apes anymore. Eventually, a Manta-shape could develop, head and eyes pointing forward, and a fleece will form between the arms and the outer legs. The tail will become less important for swimming.