As was mentioned, each fiber group has the limitation that only one of the fibers can be running at the same time. If you use seEval to evaluate a script in one fiber, you must wait for it to complete before evaluating another script in a different fiber. To get around this limitation, seEval has the SE.START flag option.
SE.START initializes an eval and then returns. Successive lines of the script are run using the ScriptEase seExec API call. Using this method, when you initialize each fiber you begin the script it is to run using seEval and SE.START, which then returns to you quickly. You do the same for each fiber. Now you can execute a single line of each script using seExec. Typically, you keep evaluating one line on each fiber in a round-robin fashion in this way. As each fiber completes its task, it is removed from the list of fibers to execute in this way. New fibers can be created and added into the list easily.