If your project is not so large and lasts for 2-3 months, you may want to release the Features for both your applications synchronously, in the same order and (almost) without delays. In this case Features and higher Epics can be kept within commonly shared backlog.
In this setup whole backlog is kept under the same multi-platform project. Tasks are platform-specific. This approach fits best for very short projects with minimal number of functionality. If your whole project can be finished within a month or even few weeks and you would like to complete the user stories in the same order and as synchronously as it is possible on every supported platform, you may consider this workflow.