couldn't it be "odešli" as well (was marked as a typo)?
Was it a listening exercise? Or why are you commenting on the Czech->English side?
No, it couldn't, obě is clearly feminine or neuter (so Obě odešla is also possible). Oba odešli would be fine.
Thanks for clarifying. I haven't taken into consideration that obě cannot be male.
And, yes, it was a listening exercise.