So the Ectoplasmatist archetype for Spiritualist states in it's Spiritual Combat ability:
At 3rd level, as a full-round action, an ectoplasmatist can make all of her attacks with a single light or one-handed ectoplasmic lash wielded in one hand, and also cast a single spiritualist spell with a casting time of one standard action as a free action either before or after making the ectoplasmic lash attacks.
The ability says you can only use the light or one-handed version of the Ectoplasmic Lash, but it does not have specific language stating the other hand must be empty/free. As Psychic spells do not have somatic components, strictly as written this would mean you can have a shield in use in your other hand.
I searched the forum far and wide and did much googling, and have not found any sort of ruling that contradicts my above interpretation. If anyone knows of any rulings of any relevance to this, I'd appreciate hearing what you've got.