Consider ALL boosting from a shard to be a "shard bonus" as well as whatever type of bonus it really is -- i.e. luck bonuses, sensory enhancement, etc. all do NOT stack -- meaning mostly it is just the spell like abilities you get, and the fact that if you were a wu jen / shaman say, one arcane and one divine shard would mean +1 caster level in both types of magic.
Regarding the spell-like abilities, the shardf itself does power each one, while the character's own abilities limit which ones he can access. So if a spell-like ability is duplicated, the bearer of two shards COULD use that ability twice, once from each shard.
If 4 shards are combined into a hemisphere, that might increase the other bonuses in some way.
-- E3