Should I ask my master's advisor and co-advisor for a recommendation letter?
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.