With the rise of cloud-based email and calendar services from Google and Microsoft the need to manually sync calendars together is receding, as most smartphones will support multiple calendars.
However, if you are looking to link two calendars together on a desktop, especially when one is hosted on an on-premise exchange, there are workarounds.
How to sync Google Calendar with Outlook
First off, if you want to sync calendars on a desktop then using the online version of Outlook - Outlook.com - automatically supports multiple calendars.
If you really want to sync two calendars though, or your hands are tied by the email exchange in your office, then you will need to export one and import it into the other.
To export Google Calendar first click the Cog icon in the top right and select Settings. Now on the left-hand menu, there is an Import & export tab, click this and hit the export button (see image below). This will download a .zip file, now copy the larger .ics file out.
Then to import this into Outlook, go to File, Open and export, import/export. Select the option to import an iCalendar (.ics) file.
To export your Outlook calendar, go to File, Open and export, Import and export. Select Export to a file and save it to a .csv file. To import it into Google Calendar, click 'Add a colleague's calendar' on the left-hand side and then select the Import option.
If you use the Calendar section of Outlook.com, you can't export, but you can import. Click 'Add calendar' then choose 'From file' in the menu and navigate to your Google Calendar .ics file to import it.
With SyncGene you can sync contacts, calendars and tasks automatically across iPhone, Android, Outlook, Gmail and apps. There is a free version which is limited to two data sources and does not offer automatic syncing, but a paid-for subscription works out at $4.95 (£3.50) per month if you sign up for a year. If you do subscribe, bear in mind there's a 14-day refund policy, so ensure you're happy before that time is up.
Sync2 syncs Outlook with Google Calendar and does a lot more besides for £39.95. It can sync on a schedule or whenever a change in Outlook is detected.
Syncing calendars on a smartphone
It's a lot simpler to sync calendars on a smartphone. So if you have an iPhone, Android phone or just about any other smartphone, you should be able to add multiple accounts (and therefore multiple calendars) and have Google and Outlook calendars appear together in the same app. This isn't syncing, but it does mean you can see two or more calendars in the same app and same view.
For iPhone specifically, the pre-installed Calendar app in iOS will display multiple calendars. Simply go to Settings, Mail, Contacts, Calendars and tap Add Account. Enter your credentials and the calendar app will automatically pull those calendars through and will even scan email for events you have tickets for or flights you have booked and pull them through too.