Is it possible/ethical for PhD students to collaborate with a professor from different institution on a topic different from their dissertation research
Of course it is! The goal of any PhD program is to develop strong independent researchers. Limiting any PhD student's collaborations to one faculty member or group would be directly opposed to that goal. I consider publication of at least one paper without the advisor to be an iron-clad graduation requirement in my field, and in particular for my own PhD students.
However, if your advisor is paying you as a research assistant, then you do have an obligation to work on the specific project for which you are being paid. You can't ethically work on another project—research or otherwise—at the expense of the work you are being paid to do. But as long as you are fulfilling the terms of your funding, what you do with the rest of your time is up to you.
Note: Your advisor may disagree with my opinion about independent work. Working on research without their explicit approval may violate their expectations for you as their PhD student, possibly to the point of them refusing to work with you further. While I believe such a response is both deeply unethical and counterproductive, that doesn't mean it can't happen.
without telling their advisors.
No, no, no. Don't even think about it.
If you feel the need to hide your research activities (the entire point of getting a PhD) from your advisor (the one person whose job is to help you succeed), then your relationship with your advisor is deeply broken. If you lie to your advisor, they will almost certainly find out later. If you can't trust your advisor, find a new advisor. If you think your advisor would disapprove of your independent work, either live with the disappointment or find a new advisor.