During your Main Phase, if "Necrovalley" is on the field: You can Fusion Summon 1 Spellcaster Fusion Monster from your Extra Deck, using this card you control and other monsters from your hand or field as Fusion Material. You can only use this effect of "Gravekeeper's Spiritualist" once per turn.
The only situation in which the Priestess-Spiritualist combo doesn't seem to work is when there is a Field Card activ on the field. While there is no Field Spell Card on your/your opponent side of the field you can use the Spiritualist-Priestess combo to summon Gravekeeper's Supernaturalist, even if they are the only cards on your field or hand. You don't need Necrovalley nor you need 2 Priestess.
If you use Priestess as material, then you'll have to send it to the Graveyard before doing the Fusion Summon, and sending it to the Graveyard removes the Necrovalley effect, thus Spiritualist's effect no longer applies.
As a result, you cannot Fusion Summon using Spiritualist's effect if the other material is Priestess.