Translation:I am driving a bus and eating strawberries at the same time.
But does there have to be? Don't recall having to specify "at the same time' in the translation when "hem...hem de..." was introduced to us in the conjunctions skill. Ektoraskan was interpreting the phrase as "both X and Y."
That said, how about, "I am both driving buses and eating strawberries"?
In my opinion your translation might be correct - at least in grammatical terms. So perhaps it is an forgotten alternative.
On the other hand 'otobüs sürüyorum' could have the meaning 'driving a bus at the moment' so that plural meaning wouldn't be possible. (not sure)
"I am both driving a bus and eating a strawberry" was accepted for me... I'm not sure what happened when you tried using singular, as it's correct. But just so you know (because it will come up a lot!), the plural "strawberries" is equally correct. :-)
If the direct object is indefinite/unspecific ("a strawberry" or "some strawberries" in general) Turkish will always use singular form. If it is definite (i.e. in accusative case) we can pluralize: çilekleri. As Marie_goforit said, we can include "bir" if we want to make it clear that it is singular: "bir çilek yiyorum".
It's a bit confusing at first, so if you want more info from a former MOD/contributor, you can read this discussion: https://forum.duolingo.com/comment/7736911
My translation, which I think is better, would be: As I drive a bus, I also eat strawberries. Here "de" is translated to "also". In English wouldn't this mean "at the same time". I'm sure English speakers would think so. In the Turkish sentence as given, the phrasing to me means that both acts are done together. Example: I know both English and Turkish. (at the same time) Hem ingilizce hem de Türkçe biliyorum! So if I were teaching Turkish I would explain that "both are done together" which in English would come across as "both are done at the same time". Check out the comment by @Yomalyn. But the feeling in Turkish is "both are done together". Sorry for the protracted expounding.