hoge.cs

var a = new ObservableCollection<string>();
var b = new ObservableCollection<string>();

var rc = a.ToCollectionChanged()
    .Merge(b.ToCollectionChanged().Select(x => new CollectionChanged<string>
    {
        Action = x.Action,
        Index = x.Index + a.Count,
        OldIndex = x.OldIndex + a.Count,
        Value = x.Value,
    }))
    .ToReadOnlyReactiveCollection();

a.Add("a");
b.Add("b");
b.Add("c");

b.Remove("b");