Concatenate two strings into one

[Colin]

@aryeduino

How can I send a value from one function to another one?

If you set msg.payload in one function and wire it to the second function then it will be in the message received by the second function.
Perhaps what you are actually trying to do is to combine information from two messages into one. If so then you can use the Join node in key/value pair mode as described in the node red cookbook example join-streams