Only one image can be loaded in NTLite at a time. If you open two copies of NTLite, each session can load its own separate image.
When a new copy of NTLite starts, it checks to see if other NTLite processes are already running, and will ask for confirmation that you really want to run another session. Each session cannot read another session's loaded image, because they're locked to preserve file consistency.
You can open two separate Explore directory folders, one from each session and go back and forth between them, since they're treated as normal Windows folders.