Quick overview of the situation, the party is 3rd level and the warlock and the bard can now both cast cloud of daggers. We were in a lizardfolk den and they saw us and came running down a 5-foot corridor. The bard was first in initiative and threw out the cloud of daggers right on them. The warlock went next and wanted to do the same. Is there any ruling against this? The lizardfolk were next and would take the 4d4 from the bard at the start of their turn. Would they also take the warlock's 4d4 (A grand total of 8d4), or do these effects need to be staggered?
It depends on whether you believe damage to be an effect of a spell, and whether you believe the cloud of daggers castings have any sort of overlapping "duration"; personally, only one instance of damage
There is the following rule from the "Combining Magical Effects" section:
The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap.
- Player's Handbook (page 205)
There is also the following optional (as every rule there is optional) rule from the "Simultaneous Effects" section of Xanathar's Guide to Everything:
If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character’s turn, the player decides which of the two effects happens first.
- Xanathar's Guide to Everything (page 77)
There are a few things that need to be determined here; firstly, whether or not damage is even an effect of a spell. One could argue that it is not (and people have done so here), in which case it does not apply to the above rule and the creature will take both instances of damage. That said, I personally believe that the spell causes the damage and therefore the damage is an effect of the spell.
There is also the question of whether the spells actually have an overlapping duration at all. The castings do have their own durations and those are counting down at the same time (overlapping). Thus the spells are overlapping and any effects they might cause must all come from only one of the castings; the creature only takes damage once.
Alternatively, you could say that the rule on overlapping effects applies to each effect separately and not to the spells as a whole; thus the damages never overlap (using Xanathar's) and both would apply fully. But this would likely be incorrect and goes against the current answer to the question "How do you choose active effects when two instances of the same spell/feature overlap?".
I personally believe that damage is an effect of a spell and that the castings are overlapping, thus the creature will only take 4d4 damage, but there is space and reasoning to argue otherwise.
An oddity from having only one instance of damage apply
Spacing apart the castings by a mere 5 feet causes radically different behavior; the creature would take damage when starting its turn and also when entering the second AoE.
If a medium creature walked into one AoE and then the other they would take the damage twice, not just once. However, a large (or larger) creature could potentially walk into both AoEs simultaneously, resulting in them only taking damage once.
The lizardfolk would only take 4d4
Borrowing heavily from this answer, multiple of the same spell never stack. From the Player's Handbook, “Combining Magical Effects”:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.
RAW, the bard and the warlock would not get more damage out of their cloud of dagger spells by overlapping them.
To narratively justify this RAW, think of it as the damage coming from the fact that the area is filled with daggers, and adding more daggers doesn't change the fact that the area is filled with daggers.
Of course, if as a DM, this seems unrealistic, you could always overrule RAW and claim that both spells do take effect, since there are now twice as many daggers filling that area.