Is it correct to a ask a question in PSE and in other sites like Physics Forums?
I suppose that there shouldn't be any problem, but I can be thought as favouring the competence, althought Physics doesn't distinguish this trivialities.
What about asking a question also in MathSE if it's very related to Physics and Maths?
You shouldn't ask the same question on multiple Stack Exchange sites at the same time. If you have what seems like a crossover question, figure out what site it fits best and ask it there. If, after several days, you're not getting a good response and the question is on topic for another site, then you can post it to that other site as well, but make sure to link both "versions" of the question to each other.
Outside the SE network, it doesn't really matter. I suppose the same procedure is useful for getting your questions answered (i.e. post it to one place first), but it's not like anybody checks. If you post a question here which has already been asked on e.g. Physics Forums, it would be useful to link to the PF thread from your post on this site. (But not the other way around, because last I checked they don't want people posting links to PSE on PF - they consider it advertising.)
