Weren't there conflicts? Who decided when to use Manchu or Chinese ?
Most official documents were produced in both Manchu and Chinese until the end of the Qing dynasty. Which meant that two copies of the same documents were made: one in Chinese and one in Manchu. However in some cases documents would be produced only in Manchu and often there would be differences between the Chinese and Manchu versions of the documents. This was necessary because the vast majority of the administrative personal were ethnic Han Chinese speakers. The Manchus would need to be versed in Chinese when conversing with Chinese scholar-bureaucrats whom they respected even before the conquest. The Manchu elite, including the emperors, started to learn Chinese and even to write classical Chinese poetry.
Over time, the importance of Manchu decreased as even the Manchu elite started to become more comfortable with Chinese than their own ancestral language. This trend is to continue until Manchu as a language became almost extinct in the 20th century.
This is a complex question and studies, like The Manchu Way by Mark C. Elliott, do not exactly address it. My feeling, based on information scattered here and there, is that the choice of language depended on:
the writer/speaker
the situation
if you were a member of the Banners (Manchu, Mongol or Chinese), you were expected to be able to use Manchu. Failure to know Manchu may lead to dismissal from office. That being said, being able to use Manchu did not mean that you had to use it each and every time you wrote or said something. For instance, some memorials by Hailanca are written in Manchu, while others are in Chinese, which leads to...
in some cases Manchu seems to have been prefered, the best example being military matters. Manchu offered more security since far less people knew it. Some Manchu documents for instance bear the mention "do not translate [into Chinese]".
But this is only a rough sketch and it cannot account for each and every case. Plus, the more time passed, the more Chinese was used regardless of any parameter.
Maybe, if you post your question at r/ManchuStudies, it will draw more answers.