Is it possible (practically, in real life) to pursue two very distinct research interests (e.g. in AI and theoretical physics)?
It is possible (even “practically, in real life”) to do all sorts of things that are very difficult to do, like: publish five bestselling novels; climb the highest mountain in every continent; win a gold medal in the Olympics; become a successful movie actor; etc etc.
The point is that questions like yours (which seem to get asked here regularly) are very difficult to give a meaningful answer to. We don’t know you and what you’re capable of. Some people can do the thing you’re asking about, others can’t. So just asking “can it be done” is a question that, if interpreted literally, has an easy answer that’s not very interesting or helpful (“yes, it can be done”), and if re-interpreted to mean what I think you really want to know (“can I , the specific person asking this question, do this thing?”), isn’t a question that I or anyone else here have the information to answer.
Perhaps the most helpful way of addressing the question is to look at the number of people who can do the thing you’re asking about. Well, I’ll be honest, that number is extremely small. Most people who want a career in academia already find it challenging enough to do one PhD in one discipline and then pursue a position as researchers in that one discipline. And even from among those super talented people who find it a bit easier than the rest and are smart and hardworking enough that they can realistically contemplate specializing in two very distant disciplines, one of which being as notoriously difficult as theoretical physics and being acquired purely through self-study, only a vanishingly small fraction of that already small group will care to invest the time and energy that it would take to broaden their reach in such a way. For those people it’s not really about a lack of ability - more about the fact that it’s an inefficient strategy for becoming successful and maximally realizing one’s potential; but it kind of amounts to the same thing in the end.
Summary: can it be done? Yes. Is it something that I’d recommend to anyone to have as their career plan? No.