Can I use the Skype for Business App SDK to develop an app that enables two end users to communicate directly with each other via instant messaging, voice or video?
this isn't currently possible. The precise answer depends on whether your end users have Skype for Business accounts:
The current SDK only supports users that are "guests": i.e. not signed in as Skype for Business users. It is not designed to allow two guests to talk to each other. Instead, it enables the guest to join a meeting with a licensed Skype for Business user who is attending the meeting via the official Skype for Business application installed on their desktop or mobile device. Example: a patient using your healthcare app joins a video meeting with their doctor who is using the Skype for Business app. For more details on this scenario, see https://msdn.microsoft.com/en-us/skype/appsdk/embedskypeb2ccomms .
In future, the SDK may support users that are signed in as Skype for Business users. This would enable these two licensed users to talk directly to each other from within your app.