"We are waiting for that boy and for that automobile."
Translation:Čekáme na tamtoho hocha a na tamten automobil.
I just made exactly the same mistake. Although in languages such as English or German the repetition of the preposition is superfluous, I think this is not so in Czech as different cases (na + genitive of masculine animate noun and then na + accusative of inanimate noun) are involved. However, it would be useful if a native speaker could confirm this.
"Čekáme na tamtoho hocha a tamten automobil." is accepted, so maybe you had a different mistake in there somewhere? They are the same case - both are accusative, only "tamtoho hocha" is animate and "tamten automobil" is inanimate, that's why they are so different.
Repetition of prepositions is quite common in Czech, but by no means mandatory. It just flows better in speech to repeat the preposition because the words stress shifts onto the preposition, so it's more harmonious - e.g. "Čekám na mámu a na tátu" vs. "Čekám na mámu a tátu." (stress marked in bold). But both ways are possible and correct.
Looks like your answer got deleted, because I have rewritten and deleted my post. Sorry, I did that at the same time you answered and I got the email notification about your answer only after pressing delete.
It is not neuter. The similarity of the word root is irrelevant, so there is really no connection with the fact that auto is neuter and automobil masculine. A related older word lokomobila (a portable engine, https://en.wikipedia.org/wiki/Locomobile ) is feminine because of the ending. Anything can happen.
Words ending with -il tend to be masculine. Foreign ones: mobil, decil, debil... Native ones derived from verbs (past participles): slídil, chlubil...