I don't think you can merge without flattening, but you can copy shapes from one layer to another.
Select the shape(s) you want to copy using the path component selection tool
[img]directselect.gif[/img]
Use the shift key if you need to select more than 1, copy, select next layer, paste.
The source of my information.....
Photoshop Help > Drawing and Editing > Editing paths (Photoshop) > Selecting paths (Photoshop)
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I thought it time i got in on the act!!!