Is it correct way to use ModelState.Remove to deal with ModelState?

I'm totally with Mr.Steve Morgan

So if your ViewModel doesn't always need some property to be Required then you shouldn't decorate it as Required.

I don't know why you wanna this issue but I suppose that in some cases you need PropertyOne to be Required if PropertyTwo has value. In this case you may need to make your CustomValidationAttribute to check these two properties.

I'm using something like this :

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public class PropertyNeededAttribute : ValidationAttribute
{
    private const string defaultErrorMessage = "'{0}' needs '{1}' to be valid.";

    public PropertyNeededAttribute(string originalProperty, string neededProperty)
        : base(defaultErrorMessage)
    {
        NeededProperty = neededProperty;
        OriginalProperty = originalProperty;
    }

    public string NeededProperty { get; private set; }
    public string OriginalProperty { get; private set; }

    public override object TypeId
    {
        get { return new object(); }
    }

    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentUICulture, ErrorMessageString,
                             OriginalProperty, NeededProperty);
    }

    public override bool IsValid(object value)
    {
        object neededValue = Statics.GetPropertyValue(value, NeededProperty);
        object originalValue = Statics.GetPropertyValue(value, OriginalProperty);
        if (originalValue != null && neededValue == null)
            return false;
        return true;
    }
}

note: Statics.GetPropertyValue(...) do nothing but get the value from the property to compare it.

Hope this helped :)