public class DynamicInputCollection : IEnumerable<int>, IEnumerable<double>
{
    private readonly List<int> intList = new List<int>();
    private readonly List<double> doubleList = new List<double>();

    IEnumerator<double> IEnumerable<double>.GetEnumerator()
    {
        return doubleList.GetEnumerator();
    }
    IEnumerator<int> IEnumerable<int>.GetEnumerator()
    {
        return intList.GetEnumerator();
    }

    public IEnumerator GetEnumerator()
    {
        // How do we deal with this common case, where in context it means two different things?
        throw new System.NotImplementedException();
    }
}