On the previous hackthis!! we were able to nest spoilers with BB code. Unfortunetaly, this feature seems to be unavailible - though this could be intentional instead of a bug. Also, information on how to use the spoiler seems to be unavailible in the markdown thread.
Example:
This input (forward slash excluded)
<\!-
Parent Spolier
<\!- Nested Spoiler -!\>
-!\>
Yeilds
Parent Spolier
<!- Nested Spoiler
-!>