In English you kill two birds with one stone when you achieve two goals in one action. In Finnish or Dutch you get two flies in one hit. Is there a similar saying in Latin? I prefer classical Latin, but any era is welcome if there is a particularly suitable saying. It is easy to translate the idioms from other languages directly, but that is rarely a good idea.
The "duo parietes" in another answer seems to have a meaning more like "have your cake and eat it too," i.e. to pursue two conflicting goals at the same time. Cicero also describes this same phenomenon in other terms in Pro Sexto Roscio, 29:
interdum mihi videris, Eruci, una mercede duas res adsequi velle, nos iudicio perfundere, accusare autem eos ipsos a quibus mercedem accepisti.
Meantime you seem to me, O Erucius, to be wishing to obtain two articles for one payment; to blacken our characters in this trial, and to accuse those very men from whom you have received payment.
Although both these expressions have the same "drift," I don't think they are good substitutes.
Plautus uses two expressions that cleave closer to the meaning of "kill two birds with one stone."
The first seems more like a literal description than an expression, but I'll include it anyway:
sed Alcumenae huius honoris gratia
pater curavit uno ut fetu fieret,
uno ut labore absolvat aerumnas duas. (Plautus, Amphitruo, I.2.24-26)
The context here is that Alcumena has been impregnated by both her husband and by Jupiter, but Jupiter has decreed that she should have both children in a single birth. The bolded portion literally means, "to complete two [birth-] pangs in one labor."
We have a second example that is actually a metaphor. It's in expression form, though there's no reason to think it was often used:
iam ego uno in saltu lepide apros capiam duos. (Plautus, Casina, II.8.40)
This can mean either "to catch two boars in one bush" or "to catch two boars in one leap." (It could be that one of these readings is impermissible: please let me know if so!)
Cicero is cited in this regard by Riddle and Freund's lexicon:
Duo parietes de eadem fidelia dealbare
Whitewash two walls out of the same pot
A proverb for one who "blows hot and cold," who "sits on the hedge," or who tries "to serve two masters."
Another possibility would be to make reference to Ovid's story of Tiresias, who struck two snakes with one blow of his staff:
nam duo magnorum viridi coeuntia silva corpora serpentum baculi violaverat ictu deque viro factus (mirabile) femina septem egerat autumnos (3.324–27)
There are caveats here too: Tiresias wasn't trying to kill the snakes, and the difficulty associated with killing two birds at once is not also found in the striking of these two snakes. But it does involve solving a "problem" by a single action, so that image might perhaps be pressed into service.