Symfony version(s) affected: 4.4.2

Description

We have a translation process where translations are downloaded from an external service then dumped to translation files.
We have two translation formats (Symfony format and icu) for thousands of translations (legacy were using Symfony format, and new ones are using icu).
These translations once retrieved were dumped with XliffFileDumper to two files messages.en.xlf and messages+intl-icu.en.xlf (each one supporting a different format).

Since #34797 was merged, it is not possible anymore to dump two files for same translation domain but with two different formats (icu and Symfony format). All translations are dumped to the same file messages+intl-icu.en.xlf as class \MessageFormatter exists.