As far as I know, it's prohibited at my university. Graduate students are expected to devote time to both research and teaching, and attempting to divide time between research and 2 separate classes necessarily means either a class or the research suffers in the end.
Well, I would see what the school has to say. But, what is the expected workload of the TA position? Typically it is about 20 hrs/week, but I have seen TA positions that barely take up 10 hours a week. So... it really all depends on the circumstances.
But, if you can live off of only having to work one of them, you should do that. The more time you can devote to your studies (especially in the first 2 years of grad school) the better.
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