За двумя зайцами погонишься – ни одного не поймаешь

With accent marks:

Translation:

If you are chasing two hares at once, you won’t catch a single one.

Meaning:

This Russian proverb addresses the problems of multitasking: doing several things at the same time will not give you good results.

The expression dates back to primitive times when hunters were stubbornly chasing just one hare until he was completely exhausted. If you chase two hares at once, giving a respite to one or the other, then soon you will be exhausted yourself and the hares will safely flee.

A close English equivalent of this proverb, although with an opposite meaning, is "to kill two birds with one stone".