For all kinds of reasons it is best to put any additional runtime on another disk. This makes upgrading to another version of Poser and to another Operating System easier. It might even improve performance depending on your hardware. Nowadays Poser itself doesn't put the runtime in the same directory as the executable (modern OS's dislike writing into these directories as executeables are supposed to be inmuteable).

There is no objection to have as many runtimes as you need and seperating runtimes in ways logical to you make perfect sense. Even placing runtimes on removeable storage is possible and sometimes even recommended for those items you don't need often.