Is it possible to write two separate Master's theses?
No one cares about your masters thesis.

Stated differently, your masters thesis will serve a single, time-bound goal: earn you a masters degree. To that extent, the amount of work you put into the thesis itself should be just enough to get that goal, and no more.

That said, you should definitely look to publish your work in as many reputable journal articles as you can. Rather than focusing on writing a second thesis, I recommend (a) writing up your first thesis topic as a paper, and then (b) writing the alternative thesis that you want to write as a second paper.

Please please PLEASE talk to your thesis advisor before taking the above advice. He or she will know whether your research is at journal-level quality and can save you a lot of unnecessary work if the answer is "no".