My advise has always been keep one productivity system. Regardless of whether an appointment or contact or task is personal or work related, you are one person living a dynamic interconnected life and bifurcating your organizational process leads to dropped items, inefficiency and in the worst cases errors and missed actions and meetings.
Well I found a use for a second calendar. In a way it isn’t even really a second calendar, but I’ll explain.
I use Google calendar within my GMail profile. Recently, when setting up this blog in fact, I realized that I could publicize my public or web-based speaking and training events in a little calendar on the side of my page. (look to the right of the screen). This widget (technical term) is fed through a link to an online calendar using an iCal URL. It is easy to copy that URL from your Google Settings but how to show my public events without publishing my entire calendar. The answer is that Google lets you set up multiple calendars with unique names and iCal addresses. I created one for my public events and linked to my website.
I still only have one calendar in terms of my view of the world because in any of the interfaces through which I view my calendar, iPhone, iPad, the Mac iCal program or Google Calendar in a browser, I have selected to view multiple calendars simultaneously (it will work in Outlook as well). I have a color code so blue is my default calendar and red is my public event calendar. When creating a new event or appointment from any platform, I choose which calendar to save it on and from there on out, the difference is moot and my calendar is still a single resource for me.
The same can applies to creating a separate calendar for your team to track vacations, business travel etc. This way you can balance resources and have access to your group schedules within your single system of productivity. The multiple calendars are like onion skin and you can turn them on or off like layers if your display gets too crowded. You may choose to create separate calendars for each conference room in your office for booking meetings. The technology allows us to keep things separate and still united.