You can link related Opportunities together by navigating to the sub-tabs at the bottom of the Opportunity screen and clicking Select Opportunity which is visible when you click on the Opportunity sub-tab.

The total value of all linked opportunities displays in the Total Linked Amount field in the Opportunity detail view.

The screenshot above shows the value of that Opportunity is £1500, the linked Opportunity has a value of £500 and therefore the Total Linked Amount is £2000.