Books split by foreign language publisher. What to do?

[DarkStar]

This happens (or used to, at least) often with that Polish publisher. They take an original work in English that is one book and split it in two halves, give them subtitles and release them as two books. It's difficult to combine them with the original work because those two books are actually one book. Argh!

Here's the original: Chasm City

Here's what the Polish publisher did:

part 1
Migotliwa Wstęga t1. Pościg

part 2
Migotliwa Wstęga t2. Odwet

They are not combined to "Chasm City" at the moment. I don't know how to do it properly and whether I should do it in the first place... Any advice would be welcome, thanks.

[Scott]

They should not be combined with the whole work.