Each trigger secret counts as a secret.
You can killtarget one trigger from another, so you should be able to have two trigger_once which point at a trigger_secret to activate it.
Which might actually work without killtargeting anything.
id1 progs is kinda rough though and I haven't used it in years so this might all be wrong :L
I'm working from a Copper base now, which allows lots of entity manipulation without having to hack the level. I have coded up similar entities in the past, but didn't want to do it again and Lunaran's are better made than the legacy stuff I was going to resuscitate yet again.
http://lunaran.com/copper/