The new Cartomancer feat from The Book of Many Things states that "You can use a card deck as your spellcasting focus." It also states that you can "imbue" a spell "into a card," after which you can "flourish the card and cast the spell within."
If a Cartomancer has donned a shield in their left hand, and is using their card deck as a spellcasting focus in their right hand, can they use the same hand to flourish the card within that deck into which they have imbued a spell?
If the spell has a material and somatic component, (*,S,M), this works, as the rules for material components state:
A spellcaster must have a hand free to access a spell’s material components — or to hold a spellcasting focus — but it can be the same hand that he or she uses to perform somatic components.
So the hand holding the deck, the spellcasting focus, can meet both the material and somatic component. However, if the spell has (*,S) components with no material component, this does not work, since the rules for somatic components state:
Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.
Since the hand holding the deck is not being used to meet a material component, it cannot be used to meet the somatic component for a (*,S) spell.
And finally, the question of "can you actually flourish the card from the deck one handed?" There's nothing in the rules that would restrict this, I typically deal cards one-handed, and I've seen card artists do absolutely magnificent things with a deck of cards one-handed. I really just don't see a reason why this would be a problem.
"*" here indicates that the presence or absence of a V (Verbal) component is irrelevant.
