I know that the usual way of using two -bogh clauses on a noun is by joining them with 'ej:

HoHbogh 'ej Qaw'bogh nuH

weapon which kills and destroys

But is there any rule which is actually being broken if we wrote the above without the 'ej?

HoHbogh Qaw'bogh nuH

weapon which kills which destroys

Why would writing something like this be wrong?

Since a relative clause is treated grammatically like a noun, doing this breaks no rules. But it's never appeared in canon, and it doesn't appear to be something that Klingons do.

Look at your English translation: weapon which kills which destroys. There's no rule in English that disallows that phrase, but it wouldn't be said in English. You'd say weapon which kills and destroys. Your phrase breaks no rule, but it's also not right.

-- 
SuStel
http://trimboli.name