Unable to display 2 markers on google maps in objective C ios

Yes, you're overriding the value. You should create another marker instead.

    GMSMarker *marker = [[GMSMarker alloc] init];

    marker.position=CLLocationCoordinate2DMake(toLatitudeDouble, toLongitudeDouble);
    marker.title=toLocationFromResultVC; marker.snippet=@"Destination";

    // Your're overriding the value here!!
    // use GMSMarker *marker2 = [[GMSMarker alloc] init]; and reference it instead in the code below.

    marker.position=CLLocationCoordinate2DMake(fromLatitudeDouble, fromLongitudeDouble); 
    marker.title=fromLocationFromResultVC; marker.snippet=@"Source";

    marker.map = mapView_;

So, the correct code would be

    GMSMarker *marker = [[GMSMarker alloc] init];

    marker.position=CLLocationCoordinate2DMake(toLatitudeDouble, toLongitudeDouble);
    marker.title=toLocationFromResultVC; marker.snippet=@"Destination";
    marker.map = mapView_;

    GMSMarker *marker2 = [[GMSMarker alloc] init];
    marker2.position=CLLocationCoordinate2DMake(fromLatitudeDouble, fromLongitudeDouble); 
    marker2.title=fromLocationFromResultVC; marker.snippet=@"Source";
    marker2.map = mapView_;