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".

This was originally a comment but it might serve as an answer:

The important thing here is your adviser to be happy with the work you have done.
If the adviser is happy with the work, even if you think it's shallow, it doesn't matter a lot given that your adviser will write a good recommendation letter so that you can be admitted to the PhD program. Of course a good publication can help, but it is not always necessary.

I understand that you might think that a master thesis is a "preparation" for the research you will do during your PhD. At least in my field, you won't dive immediately in research during your PhD. You will have a lot (depending) of time to study related material that you feel you missed because you should/could learn that on the Master thesis. From my point of view, you won't have any problem given your motivation.

So, I suggest, try to make the best out of your current thesis, while talking at the same time to your adviser about potential issues regarding your PhD application. If they are happy with your work, it will be reflected with a good recommendation letter which is the key for getting into a good PhD program.

Can you write two separate theses for the same degree? Probably not. The regulations for your course probably implicitly require you to write exactly one and, even if they don't, the department will feel under no obligation to grade a second thesis.

Writing a second thesis makes no sense. If the thesis you're working on at the moment really is inadequate, there are two possibilities: either you make it good enough or you abandon it and start a new one on a new topic. If you make your current thesis good enough, there is no need for the second one. If you start a second one, continuing to work on the bad one is a waste of your limited time and you'll end up with two bad theses instead of one.

You need to talk to your advisor about your situation. It is literally their job to... advise you.