I don't really know much about MA programs in the US but I was in a fairly similar situation, so here's my two cents.
Last year I finished an MA in philosophy at a university in India, which I'm pretty sure none of the philosophy departments in the US must have ever heard of. I did have decent GRE scores (V167/Q165/AWA5.0) and good grades but like you I was quite apprehensive about my chances of getting admitted to a PhD program because I came from a university that is not well known. But the results were in the end not all that bad- I got admitted to Mizzou and I was waitlisted, though ultimately rejected, at UVA and Minnesota, out of 6 total applications.
So based on my case alone I would be willing to bet that you do not need an MA from an american university in order to get into a PhD program in the US. Though whether you can get into a 'good' PhD program would be more tricky to say; I think an MA from an american university would improve your chances . Also, in most programs you earn an MA on your way to the PhD anyway, so whether you wish to do an MA before that is something I personally would consider seriously, given that you are already doing an MA. I don't see any reason why you wouldn't be allowed to apply for a second MA, particularly since its a different country, but my guess would be that spending time working on your writing sample would be an equally good way to improve your chances of getting in the next season.