2. You always use markers[0] and TARGETS[0], so you create only one marker.

LocationService GPS;

    OnlineMapsMarker[] markers;

    void Start () {
        GPS = new LocationService();
        GPS.Start();
        foreach(var t in TARGETS) distanceList.Add(0f);

        MakeMarker();
    }

    void MakeMarker()
    {
        markers = new OnlineMapsMarker[TARGETS.Count];
        for (int i = 0; i < TARGETS.Count; i++)
        {
            markers[0] = new OnlineMapsMarker();
            markers[0].texture = markerTex.target;
            markers[0].position = new Vector2((float)TARGETS[0].longitude, (float)TARGETS[0].latitude);
            markers[0].enabled = true;
        }
        OnlineMaps.Instance.AddMarkers(markers);
    }

OnlineMaps.AddMarker
http://infinity-code.com/doxygen/online … 4123f84113

Example:
http://infinity-code.com/atlas/online-m … ample.html