You can launch one app, put it in background and launch other app using start Activity and bring the background app back
Something like this
Try to focus on the current activity you were in :
driver.runAppInBackground(10);
driver.currentActivity();
or
Try to start the activity you were in :
driver.runAppInBackground(10);
driver.startActivity(“appPackage”,“com.example.android.apis”, null, null);