Q:

Travelling using two passports of the same country

I am German and I am holding two valid German passports.
My first passport (Passport A) contains a Chinese visa.
My second passport (Passport B) is still empty so far.
Now I want to travel from Germany to Singapore and move on to China from there.
My plan is to fly to Singapore and enter with Passport B.
When leaving, I need go to China with Passport A (as it contains the visa - I can clear immigration with passport B in Singapore).
Are there any problems about this plan, at the check-in in Singapore, immigration in Singapore or immigration in China?
If I check-in with Passport A, I can't prove legal immigration to Singapore (as the stamp is in Passport B)
If I check in with Passport B, I can't prove I hold a valid Chinese visa).
Will the problem be solved by presenting both passports?
Which passport should I book the flights on?

A:

While I haven't been in the exact same situation, I have two German passports as well and I have been travelling in Asia for the last few years using both of them.
Pretty much always, use the same passport to exit a country that you have used to enter it. Using the second passport when arriving in the next country should be fine. If they really want to see prove of your stay in the previous country you can show them your second passport. 
Some tips:
In some(many) countries it is illegal for locals to own two passports of their country, so immigration officers may be confused, but you can tell them that is is perfectly legal in Germany.
If you plan to apply for a third country visa in a foreign country, you may have to use the same passport that you used to enter that country, example is applying for a Chinese visa in Hong Kong. 
In general keep your second passport away from the immigration office. When entering Turkmenistan from Iran, I handed in both passports because one had my Iranian exit stamp and the other one my Turkmen visa. The border guards were completely confused because they didn't realised both passports belong to the same person and complained about the missing Turkmen visa in the first passport, I just explained to them, they are both mine and all was fine.
One of the reasons why you can have two passports in Germany is to allow you to get many visas in a short period by sending them off to embassies in parallel, so you may end up with some visas in one passport and other visas in the other one. In my experience it was perfectly fine to 'switch' passports between border gates (or on the plane)

