Use a multithreaded multimanager, and another multimanager triggered simultaneously with the original trigger of the actual shake-causer, as so:
1. multimanager "shaker"
2. env_shake "shake"
3. multimanager "killer"
4. trigger_relay, killtarget = "shaker"
Trigger 1 and 3 simultaneously.
1: Key "shake", delay 0.1,
1: Key "shaker", delay 5
2: Length 5.05s
3: Trigger "shaker" after 20s.