Items such as Vana'clock and Prismatic chest require multiple duplicate copies of the item to obtain the key item effects (if you have other furnishings). This means more inventory space, more disruption for your decorations.

Why not allow these items to be merged via synergy to create +1, +2, +3 variations.

This would allow a Vana'clock +3 to replace the 4 vana'clocks currently required, similar idea for prismatic chests.