1) Enable autostart for 1 thread only (set others to disabled)
2) Start other threads using tx_thread_resume().
3) You will need to create external linkage with the thread handle to make it visible from other threads (i.e. extern TX_THREAD gui_thread), as tx_thread_resume needs a pointer to the thread control structure in order to resume it.
A good example of how this is done is found in our GUIX Audio Player application note available on the Renesas web site- here