Assigning advantage and disadvantage is the purview of the DM. Page 239 of the DMG reads, in part:
you [the DM] decide whether a circumstance influences a roll in one direction or another, and you grant advantage or impose disadvantage as a result
Does this mean the DM should (or must) choose either one or the other for a given circumstance, or can the DM choose to have one circumstance simultaneously apply advantage to one party and disadvantage to the other?
The books make clear that one circumstance can apply a (dis)advantage. It's also explicit that multiple circumstances can apply to a contested roll, resulting in possibly multiple parties receiving an advantage and/or disadvantage. I'm interested in whether or not a single circumstance can simultaneously impose advantage on one party and disadvantage on the opposition on the same check.
The example situation in the book on the following page gives multiple examples of (dis)advantage in a wizard-vs-ogres situation. However, all of these examples are one party vs. the environment or some other context, not compared to the other party.
Here's one situation where a contested roll might have ambiguity on which party should get the (dis)advantage [which leaves open the option that they both get it simultaneously from one circumstance]. A and D are wrestling. A is much larger than D. Does A get advantage, D disadvantage, or both?
Best Answer
Yes, A DM could impose disadvantage and grant advantage on the same contest.
There is precedent for it. The Cloak of Elvenkind grants both advantage and disadvantage to a Wisdom (Perception) vs Dexterity (Stealth) contest. From the SRD:
It is therefore possible to imagine other situations in which a Dungeonmaster could grant similar concurrent bonuses/penalties. Note that items which do this are very few. A ruling such as this based on a current situation should be just as rare. Advantage vs Disadvantage contests are pretty much a foregone conclusion, and almost beg the question "why bother to roll?"