As Linux distributions are entire operating systems, you cannot run them side by side on the same hardware; the OS takes control of the hardware, and the hardware requires this type of software to distribute its resources. So the only way to do it is by either running the distro B in a VM on A, or use another operating system specifically designed to run other OSes side by side on top of it. This will necessarily be in VMs, though. I think there’s a Linux distribution built for this (in the name of security), but I can’t remember the name. I could post this as an answer, but the question seems like an XY problem to me; what you actually want to accomplish, is running only parts of distro B in distro A, natively. That I assume would exclude B’s kernel?

If I’m correct in my assumption about the question, would it be a good idea to split this question into two questions, one asking to literally run two OSes on the same hardware as equal entities (has to be a beginner question) and one for running specific parts of one distro inside another distro (advanced question)?

I guess this is a rather related question over at SE: https://superuser.com/q/333297