Double Link was previously part of the Denali for Salesforce "secret menu," but now it's official for anyone who wants it. This feature allows a call to be associated with both a "who" object (Lead or Contact) and a "what" object (e.g. Account, Opportunity, etc.) with a click of a button. The most common use case would be to associate a call with both a Contact and an Opportunity.
Double Link is enabled on the back end for your account. Currently there is no front end indicator that the feature has been enabled.
Using Double Link is simple.
First, place your call to the person you need to speak with. Then, while on the call with that person, navigate to the second record that you want to link the call to and click the Link icon:
The name inside the dialer panel will update to show the name of the second linked record:
Complete the call and select the appropriate Disposition as normal. After completion, the call will be associated with both records. In the Standard Package, the association will show up on the Task in the "Name" and "Related To" fields:
In the Enterprise Package, the association will appear in the appropriate Object fields on the DialSource Action Detail:
If you're unsure which package you have, check out this article.