When I place 2 ladders & want to use source & destination arrows to identify the vertical buses, I fist place a source arrow on the lower end of the first ladder's left-hand bus...I place the corresponding destination arrow at the top end of the next ladder...I place another source arrow at the lower end of the first ladder's right-hand bus.
When I go to place the second destination arrow, I get a message stating "Signal Destination arrow already present -- two are not allowed on the same network."
I should be able to place this second destination arrow...Why does it stop me?
Electrical is preventing you from creating a "dead short". For example, you cannot feed both L1 and L2/N or 24V+ and 24V- to the same circuit. The rungs on the ladder are no longer "dumb" lines like they were with plain AutoCAD. They are current-carrying conductors. Electrical sees the short circuit and prevents it. Once you insert components into the rungs, removing the dead shorts between the left rail and right rail, the final destination arrow can be inserted.
This is a common pitfall so I intentionally allow students who attend my AutoCAD Electrical classes to attempt the same thing you did, so they can see how Electrical works to prevent the collision of opposing electrical potentials.
Tip: You can link one source arrow with multiple destinations, but you cannot link multiple source arrows to the same destination.