It's called a parasitic twin, siamese twin is where there are two whole bodies stuck together(sometimes missing an arm or leg or something or attached body to body, just a part of another is called a parasitic twin and one is not really so much alive as a not right part of the body that shouldn't be there and wont survive without it), though normally one is not alive it just feeds off the blood and life, though I have seen animals and humans born with four legs and the legs worked fine...that's why it's called a parasite since it still uses the bodies energy and blood like another normal body part would. I've never heard of it happening with a fish before though. Normally on another animal it'd be surgically removed if it was possible to do so.
I'm not sure what to tell you to do about it, if it seems ok and does as it should(eats, swims, poops normally) you could leave it be. It'll likely have a shorter life span. If it seems to be having issues or is unable to poop(sometimes the anus fuses and they can't) it'd be best to put it down, but only if it's having issues. Otherwise the choice is yours.
Chances of survival are unknown, they do typically have a shorter life because they expend their bodies resources faster, but unless it's cutting something off that the main body needs to survive, it should live for a while. Quality of life depends on the animals own will and again where it's located and what it's taking.
