Aside from other conversation, I’ll post my take without attempting to reconcile the previous comments to my view.
In the case of a Conflict in which both sides are reduced to Dispo 0 in the same action interaction between one another, they both lose. Not, they both win. So, this is a bit different perspective, and I’ll use your example of Deliver the Mail for my suggestion.
The mice want to drive off the raven; the raven wants to get that mail away from the mice. The goals are fairly good; it is not an exact denial of one another. In case the mice must compromise, they can offer that the raven is driven off with the mail rather than driven off before getting the mail. In the case of a compromise, the raven can offer that it refuses to be driven away, escalating the scenario to something more aggressive and potentially deadly to these stubborn mice. The goals are fairly good.
But, having both sides reduced to zero in the same action means both sides are losers.
Now, I must admit the rules clearly state that both sides get what they want, so my suggestion is that they get what they want from a losing approach rather than a winning approach. The GM will have more authority to twist up the results with unapproachable issues–the raven grabs the mail from the mice, and gets driven away. In the course of taking flight, it slips out over a gully when the bag rips, spilling most of the mail across the terrain. The mice have driven the raven away, but lost the mailbag and the mail. Some of the mail has been scattered with the winds over a large terrain piece while some might remain with the raven. The raven got only a portion of the prize, and probably knows no better whether it has something of value.
Later, in the design of Deliver the Mail, there is an NPC awaiting mail from this delivery. If it is not delivered, she might attempt solo travel–a dangerous endeavor. This can be further twist in the outcome–they’ve clearly lost the envelope for her, and she is not going to ask for help.
The outcome is a good bit worse looking at it from the perspective of both sides approaching as losers rather than both as winners. Yes, both sides in a tie get what they want, but in the least desirable way.
Such an outcome also invites the players to consider whether and how they gather the scattered mail. Should they? Or might they report the loss with no further effort to recover the mail? What if they go after the raven first? Would something happen to make the case of the scattered mail worse? Like, what if there is also incoming storm clouds rolling overhead? How far would the raven go before realizing the mail is less enticing than the shiny buckle of the mailbag? If they pursue the raven’s roost, might they find the only prize is the shiny buckle of the mailbag, and not the remaining mail?