Once you've submitted your first fan works, you are free to claim second prompts through 30 April. The deadline for second fan works is 31 May, just as it is for first ones.
To request a second prompt, email me the number of any unclaimed prompt in the prompt-claiming post when you submit your first fan work. :D
If you've questions about claiming second prompts, feel free to ask them in reply to the sshg_pf_mod's welcome post, or email them to sshg.promptfest [at] gmail [dot] com.