Union two ObservableCollection Lists

The LINQ Union extension method returns an IEnumerable. You will need to enumerate and add each item to the result collection:-

var unitedPoints = new ObservableCollection<Point> ();
foreach (var p in observableCollection1.Union(observableCollection2))
   unitedPoints.Add(p);

If you'd like a ToObservableCollection then you can do:

public static class MyEnumerable
{
    public static ObservableCollection<T> ToObservableCollection<T>(this IEnumerable<T> source)
    {
        var result = new ObservableCollection<T> ();
        foreach (var item in source)
           result.Add(item);
        return result;
    }
 }

Now your line is:

var unitedPoints = observableCollection1.Union(observableCollection2).ToObservableCollection();

Do you want to merge the existing contents, but then basically have independent lists? If so, that's relatively easy:

ObservableCollection<Point> unitedPoints = new ObservableCollection<Point>
    (observableCollection1.Union(observableCollection2).ToList());

However, if you want one observable collection which is effectively a "view" on others, I'm not sure the best way to do that...