Stacking Disadvantage (5e Variant Rule)
From D&D Wiki
A player-character can have disadvantage multiple times on an attack roll, ability check, or saving throw when the sources of disadvantage are significantly different. Having "double disadvantage," for instance, would involve rolling three d20s and using the lowest result among them.
A DM is encouraged to negate one instance of disadvantage if its source is effectively the same as another source of disadvantage. For example, making an attack against an invisible creature while you are blinded might only count as one level of disadvantage, as the major hindrance in both cases is the inability to see. By contrast, if a character is nauseated from the poisoned condition and can't see, these two distinct sources of disadvantage would stack.