Spending time / estimation in Gantt Charts

I have an issue. In our project we have 2 teams working simultaneously (Android developers and iOS developers).

When I create an epic and then I create the subtasks (separately for iOS and Android) the spending time of the epic sums up. 

What in consequence meant that the spending time on the timeline is doubled. 

Example: There is an epic to develop splash screen of mobile application. 
iOS developers reported 1 md in their issue, and Android developers reported 1.5 md in their issue. 
Total spending of the epic equals 2.5 md.
But on the Gantt chart, it should be displayed as 1.5 md, because the work was completed after 1.5 md.

I would appreciate any help.