If user's action effects something in a different space, what are some intuitive ways to illustrate this to the user without taking them out of their workflow? Right now when a user acts on the item, it is added to a sibling's page and reflected as a number next to the menu item. My concern is if the user isn't staring at the secondary menu while the action is taking place, they may miss it. How do I remedy and give the user the appropriate feedback?
I would use highlighting and a flash message. Choose your colour carefully to stand out and be friendly to the various types of colour blindness ...
On action completion show a flash message, or for more serious actions, show an 'are you sure' message.
Also, calculate where else the action has changed things, mention that things have changed elsewhere and highlight those other places with the same colour as the flash message.
If the effected places are on the screen then border them or change the background colour (be sure to check text is still readable). If the effected places are deeper in the system then highlight the menu items under which they sit.
Leave the highlighting in place until the user clicks dismiss or visits the place where the change has taken place.
For an interaction with more serious consequences (like a permanent delete) you can perform this highlighting step between the user choosing the action and confirming the action.
In a shot answer: Feedback. Show the user some feedback when he does the action, something like a popup message in the top right corner that stays there until he closes it, or even fades away in a period of time.
And also consider on prompt the user about the action that he is about to take.
Just make visual feedback. But I'd recommend not only flash message. The user should not keep in mind which items were added.
If the user actions affect something in the other space,