I have a compound sentence:
It’s bad, and we should not do it.
How can transform it into direct question? Should I invert the word order in both parts of the sentence or only in the first one?
Is it bad, and should we not do it?
is grammatically fine, and a direct inversion that can mean pretty much what you want it to mean. It sounds odd, though, because negative interrogatives like should we not? (and also the contracted form shouldn’t we?) are idiomatically used for affirmative statements, roughly translated something like
I think we probably should, and want you to say whether you agree.
This usage is so ingrained that I have a hard time "translating" without using another negative-positive; other interpretations would be:
I think we should, don’t you agree?
Don’t you think we should?
The compound question at the top is unlikely to be interpreted this way, because of the first clause, but it still just doesn’t sound quite idiomatic.
To get around this, we need to rephrase the second part of the question in a more unambiguous form, or restructure the question altogether. The first possibility would be to simply remove the negation:
Is it [good or] bad, and should we do it?
Notice that this question does not presuppose an answer, so we possible answers would include:
- It is bad, and we should not do it.
Opposite of original:
- It is good/not so bad, and we should do it.
Something in between:
It is bad, but we should do it [anyway].
It is good, but we should not do it [anyway]
Alternatively, we could leave the order of the words in the second part so that we aren’t making it into a negative interrogative. This also requires changing the way that the two parts of the sentence are connected:
Is it so bad that we should not do it?
Is it bad, and therefore we should not do it?
These versions come close to the meaning asked for in comments, is it true that it is bad and we should not do it? However, in the original compound sentence any causal relationship between the two clauses is only implied, and in these versions it is made explicit.