Both idealism and materialism have convincing arguments for me, yet within each
I'm not sure what arguments you have in mind for these two views, but I understand, I think, the main feature that raises your question: each implies that the other is false. And yet, as you say, you find that each is equally likely. If each implies that the other is false, then they can't both be true, to answer your last question. One of them, at least, must be false. Perhaps they are both false, if they both rest on a false presupposition, for example. This seems to be the situation with the two views, (a) that the present king of France is bald and (b) that the present kind of France is not bald. They contradict each other, so one, at least, must be false. In fact, it seems they are both false, since there is no present kind of France at all--though I should note that, at one time, more philosophers thought the two views lacked a truth value at all (that is, they were neither true nor false, so on such a view we could say that the two theories you are considering might lack a truth value at all).
So much for that last question, whether they can both be true. They can't. However, there's another interesting question in what you wrote. If you find both theories equally likely, and assuming that there isn't some third theory that is also a competitor, then why not just be equally confident in one as you are in the other? For example, if you think that Idealism and Materialism are the only options, and they are equally likely, then you should think there is a 50% chance, or probability, that each of them is true. This is what you would do when it comes to a coin flip. So why not split your confidence in this way with these theories? Not every contradiction that is unsettled by our evidence is a paradox. Sometimes it makes sense to simply be as confident as our evidence calls on us to be, and no more, in each option. This doesn't mean stop thinking about it. You may, for example, soon discover that the case for Idealism is not as good as you are thinking it is now, and that the case for Materialism is actually better. So you'll get more confident in Materialism.