Thanks for spotting that, English 1b3.
He said there was an attempted robbery at the store two weeks earlier and a $160,000 robbery late last year.
I see this as a compound sentence comprising two main (independent) clauses but by a process called ellipsis, part of the second clause has been omitted (to avoid repetition). Once the ellipsis has been ‘filled out’, the sequence 'and a $160,000 robbery late last year’ can stand as a main clause:
[He said there was an attempted robbery at the store two weeks earlier] and [he said there was a $160,000 robbery late last year].
From that, you can see that there are two independent clauses (shown in square brackets), each having its own verb/complement agreement. In that particular sentence, each clause has singular verb/complement agreement, but it needn't necessarily be like that:
[He said there was an attempted robbery at the store two weeks earlier] and [he said there were two robberies late last year].
Sorry if I missed the original poster's point.
Best
BillJ