I disagree. Having double the slots in a dual lightsaber is no different than a Sentinel that weilds a light saber in each hand.
The main purpose behind the crystal is to provide a stat boost. Sentinels use two sabers, so they get two crystals. Shadows use a double bladed saber and a focus item in the off hand.
Both classes get the same amount of stat increasing items.
If a DBLS used two crystals, you'd have to cut the bonus in half, or otherwise the shadow gets 3 stat increasing items.