I have dual German/Brazilian citizenship. Brazilian citizens are allowed to travel to Russia without visas, Germans are not. If, at the border, I presented both my passports without a visa, would I be refused entry to the country? Is it forbidden or immoral to present only one of my two passports?
As a rule, on entering a country you need to produce a valid travel document. There's no requirement to present more than one travel document, or every travel document you possess. You present your Brazilian passport at a border control, and that is probably all.
Same as you don't need a long-term Shengen visa in your Brazilian passport to live in Germany.
I would encourage you to only show one passport to Russian immigration authorities. If you have several passports, it could raise suspicion that you have fake travel documents. I would avoid any confusion for them, especially if you don't speak any Russian.
I would only reveal what is required of you, the Brazilian passport. If you don't need your German passport, leave it at home, as you wont want to lose it in Russia.
Also, your statement...
"Brazilian citizens are allowed to travel to Russia without visas, Germans are not."
is only partially true. According to the Saint Petersburg travel info website:
"Brazil (for visits of up to 90 days in any 180-day period) - for tourists, private visits or transit purposes only. In all other cases, a visa is required. "
If your purpose is tourism, you are fine. If you plan to work or study, you will still need the proper visa, regardless of your citizenship. You didn't really specify your purpose of travel.