I am trying to use Portfolio to plan our development timelines and ensuring effective resource utilization. However, facing issue as resource utilization is not as per our expection.
We are developing mobile apps (iOS, Android) with app Backend (web services). The estimates for each tasks are already entered in Portfolio.

In above example "Wishlist" is user story, with sub-tasks for each of iOS, Android and Backend. The estimate for iOS and Android tasks is 7 days, however estimate for Backend is 2 days.
Now as per my understanding, the Backend guy should start on next user story "Saving Payment Cards", after 2 days, and iOS/Android guys should keep on working on "Wishlist". But according to portfolio, the backend guy will start working on next feature after 7 days (when iOS/Android are also finished). Which is actually not correct, as he would have already finished his work in 2 days and will be sitting idle.
Is there any why to effectively use portflio in such scenerio?