Should I ask my master's advisor and co-advisor for a recommendation letter? I'm doing a master's, and I plan to start a PhD next year. All the admission processes I've checked so far require two or three recommendation letters. My question is: should I ask my advisor and co-advisor for them? On the one hand, they are the ones who know me the most and who certainly have the most writing material for the letters. On the other hand, I've been wondering: if I get a letter from each of them, could this be seen as biased by the evaluators? Also, another related question: my co-advisor has a PhD, but is not currently employed as a full-time professor (he recently finished his postdoc). Should I only ask professors for LORs, or is it OK to ask him too?
It's absolutely fine to ask for reference letters from both of them. Be aware that they may be somewhat redundant if you worked on the same project with both of them. So if referee A worked on project 1 with you and referee B worked on project 2, it would be much better than if both worked with you on the same project. 

To clarify - it is expected that you get letters from your advisors. In fact, not getting letters from the people who worked most closely with you in an academic setting will send all sorts of wrong signals to people reading your application. They may assume that you fell out with them or had some other issues that prevented them from recommending you.