Time allocation for resources in a Sprint

[susan]

I am executing a project with a 2 member team. I have 1 backend developer and an app developer. I have created stories (and sub tasks as per requirement) in Jira (I'm using the free version of Jira currently). 

As part of Sprint 1, Backend developer requires 10 days(initial framework setup, database setup, writing initial set of APIs etc) and app developer requires only 4 days(initial setup, screen design, API integration etc).

In Sprint 1, the app developer requires less amount of time than the backend developer. Is that the correct practice to show resource as idle for sometime ? or can I assign tasks of "Sprint 2" to app Developer in Sprint 1 itself? (I cannot more stories in Sprint1 as it will increase the work for backend dev also).

 What is the best practice to be used in Jira in this case?

Many Thanks!

Susan.