You can open multiple Preview windows with different jobs, but all jobs must come from the same Fiery Server. You can edit the jobs in each window separately. You can also drag pages from one window to another.

When you drag a page from one window to another, the source window (from which you dragged the job) becomes Read Only. The destination window is editable, but you must save the new job with merged pages in the destination window before the source window becomes editable again.