They can't be hidden in PS23. They can be grouped in a single thumbnail in PS24 and later ungrouped.
Thanks Saby,
Another good reason to get PS24
Grouped : http://forums.pinnaclesys.com/forums/storage/251/802474/10-08-2020%2023-33-10.jpg
Ungrouped (classic view) : http://forums.pinnaclesys.com/forums/storage/251/802475/10-08-2020%2023-33-42.jpg