You may think that joined and conjoined are the same in the beginning, but when you say join, this means that you are going to join more than two things together so that they will be one.
When you say conjoin, this means that there may already be things that are combined and they will be united and combined further. If you would check these words in the dictionary, you will see that there are also some differences with their dictionary meanings.
When you say conjoined twins, this means that the twins are joined together and may need to be separated. When you say joined, these are the twins that are already together in the uterus.