Is it a good practice to work on multiple projects simultaneously during PhD study?

This is very subjective and environment-dependent, and my opinion is orthogonal to qsp's answer based on my experience.

My PhD topic was a continuation of my MSc topic with the same advisor. However, halfway through I found another topic, unrelated to the first one, that really interested me. Long story short, I ended up with a sandwich thesis (this was planned from the beginning) consisting of 3 published papers on the 1st topic, and having published 8 papers on the 2nd topic to fill my CV (plus 2 papers on a third topic from a completely different field, and 2 other dydactic texts in lesser peer-reviewed journals). This was enough to graduate with honors (1st topic) and to get a prestigious grant on the 2nd topic which is my main research interest now and on which I'm building my scientific career. I'm very pleased with this outcome. (I abandoned my 1st topic altogether as eventually I lost interest in it.)

So if anyone asked me if they should work on unrelated topics during their PhD I would without hesitation advise to go for it: one broadens their knowledge (some ideas from one topic may be useful for the other), builds a bigger professional network, learns to work on different things in parallel, and ― like me ― may find a topic that is a much better fit than the original one. And I think it should be considered an advantage, as it shows one is open-minded and flexible ― and it was by my thesis referees and the grant committee.

At least at my institute, it's rather common that PhD students are involved in a number of projects on different topics, lead by different senior scientists, and also conducted by themselves. One of my friends is actively involved in ~5 of them, and keeps doing well in each.

But, as I said in the beginning, this is subjective. Some people may simply not be suited for it; some advisors may dislike it (having a conflict with one's own advisor is almost always a bad idea; on the other hand, a PhD is a time to build one's independence and scientific maturity, but this is a different issue); if someone is in a short (e.g., 3 years) program with fixed duration, indeed it might be a better idea to focus on one topic for the time being (I did my PhD in 5 years); if someone loves their topic, why should they look for something else? Etc.

So one should judge by themselves if it's a good thing to work on multiple projects. This is advisor/institution/field/culture/perspectives/... dependent, so there's no one universal "yes/no" answer.

IMHO, it is not a good idea to work on completely unrelated projects during PhD.

• If your thesis will be a monograph, how will you tell a story that connects two unrelated papers? You may need to leave one out, and the contribution of the thesis is thin.
• When you get stuck and you turn to the second project. A likely result is that the first project will be abandoned forever. Procrastination, in any form, is never a good thing.
• Suppose after finishing PhD, you have 6 papers: 2 in each topic A, B, C. You want to apply for a postdoc in topic A, how would you compare yourself with another applicant with 4 papers in A.
• You are considered an expert in a research direction only when you keep pursuing it. Publishing one or two good papers then moving to another direction is considered street smart, but never got the respect as an expert.

Broadening your research and working on different project is only good after you become more senior, e.g. postdoc, and have several junior collaborators to work with.

You don't say where you are in your studies, but I'll guess near the beginning. If that is true I'd suggest that you keep a few things in mind.

Normally doctoral studies are about depth not breadth. The exceptions are rare. You delve very deeply into a very narrow subfield of the larger field. You want to make a significant contribution there. If splitting your time leads you away from that goal it will not serve you well.

However, you don't need to turn your mind off to all other thoughts while you pursue depth. Of course you can think about other things and when the going gets hard on your main topic it is sometimes useful to turn to something else that is related or not.

You can, in fact, carry on a long term relationship with these alternate thoughts, returning to them on occasion without losing contact with your main study. Your dissertation won't last forever. If you do it well you will have the opportunity to then follow up intensively on the other things you've been thinking about.

But keep notes about those other thoughts so they don't dissipate as you progress toward the degree. When I finished the doctorate (math), I had a drawer full of thoughts (and minimal progress) on other things that it would be worthwhile to pursue. Since these were somewhat related to my main thesis they actually gave me a deep knowledge of a somewhat broader field than my dissertation covered. Still just a subfield, but an important one.

Focus on your main study. Let your mind wander. Don't get confused about or distracted from your main task. Take notes.