To make XOD programs behave predictably, there are some rules on how pins can be linked.
You’re not allowed to link two input pins, nor are you allowed to link two output pins. It doesn’t make much sense.
A link must always connect an output and an input.
If you want to mirror values, just create multiple links from an output pin.
An output can have an arbitrary number of links, but an input can have no more than one incoming link.
If an input and an output have the same data type, they may be linked as is.
However, if they have different types, they can only be linked if the output type can be cast into the input type.
Once you start linking, pins that are suitable for the other end of the link are highlighted.