Where do you add a second address for a google local result?

My acquaintance works for a company in town 1. Their domain shows up in the map results if you search in that area.

But she works in another town at a shared office space. What do you do to get that one to show up for that city, despite there being many other businesses at the office?

You can just add the business with the shared space address. Its okay if multiple businesses have same address until and unless they have the same phone numbers.The later can cause you a problem. Google says/supports if their is staff available at the mentioned working hours.Then there wont be a problem.

Check this thread and couple other similar threads on the forum.Hope it helps!