I was reading about apologetics the other day and read as part of the causal argument for God's existence that there cannot be more than one necessary being (cause), meaning that there is only one God, for a being that is so perfect to the point of being necessary must be wholly perfect. I also read that Avicenna (I think) said that a necessary being is pure existence, meaning that existence is the only thing there is to its being, and that multiple necessary being would be identical and, therefore, one.
However, I've still been struggling for solid couple months to understand why the fact that there is only one God follows from the fact that first causes, or beings, are necessary. What is the connection between there existing only one God, necessity and identity of multiple necessary causes, if multiple? Why would multiple necessary beings need to be identical and, therefore, one? I've looked into the Stanford Encyclopedia of Philosophy, Roger Scruton's lectures, William Lane Craig's works and just plain Google, and couldn't find the answer. A similar but slightly more general question was asked here, but the answers were related to simplicity of argument, questioning its validity, or about things we encounter contingent things in everyday life, none of which actually helped me.
Thanks in advance!