Is there a time where I want both Libro Mortuos and Graveyard?
You could keep Libro as a contingency plan in case Graveyard is destroyed.
Otherwise, you usually won't have enough mana to profitably use both. If you want a large swarm fast, it is generally better to just use Libro and your full action to summon them. By this method, it is possible to have 8 [mwcard=DNC24]Zombie Minion[/mwcard]s/[mwcard=DNC19]Venomous Zombie[/mwcard]s by turn 5:
Turn 1 (20): [mwcard=DNQ02]Death Ring[/mwcard] (5) + Libro Mortuos (9) = 6
Turn 2 (16+1): Zombie (6) + Zombie (6) = 5
Turn 3 (15+1): Zombie (6) + Zombie (6) = 4
Turn 4 (14+1): Zombie (6) + Zombie (6) = 3
Turn 5 (13+1): Zombie (6) + Zombie (6) = 2
You can use the extra 2 mana for a [mwcard=MWSTX2FFE04]Brace Yourself[/mwcard] if your opponent rushes. By comparison, you cannot get a similar swarm as fast using Libro+Graveyard(+Meditation amulet, generally) because you just don't have the mana.
If you want to use both, it will take you longer to set things up so that you can actually use them. One possible strategy that could work is:
Turn 1 (20): [mwcard=DNQ02]Death Ring[/mwcard] (5) + Libro Mortuos (9) = 6
Turn 2 (16+1):[mwcard=DNJ04]Graveyard[/mwcard] (14) + [mwcard=DNC23]Zombie Crawler[/mwcard] (3) = 0
Turn 3 (10+2): [mwcard=DNQ10]Meditation Amulet[/mwcard] (4) + [mwcard=DNC15]Skeletal Minion[/mwcard] (4) + [mwcard=DNC15]Skeletal Minion[/mwcard] (4) + Meditate(+3) = 3
Turn 4 (13+2): [mwcard=MW1C32]Skeletal Sentry[/mwcard](7) + [mwcard=DNC08]Unstable Zombie[/mwcard] (8 ) + Meditate (+3) = 3
After turn 4, you will have 15 mana to spend per turn and 2 discounts, which is a respectable amount. You can summon smaller skeletons to protect you when under threat, and larger zombies when reasonably safe. Your quickcast can be used to cast [mwcard=MWSTX2FFE04]Brace Yourself[/mwcard] repeatedly on yourself for protection.
Now that I think about it, the latter strategy isn't so bad after all; you just need to mix skeletons (defensive) and zombies (offensive) to protect yourself while you build up.
Makes me want to build a new necro spellbook.