Multiple NPCs and act_busy

I've figured out how to make act_busy work for at least one NPC, but any more than that and it doesn't work for all the NPCs.

I have tried these tutorials, with no luck:
http://www.interlopers.net/tutorials/31744
http://www.youtube.com/watch?v=xKHqa_q6UB4


I cannot find any tutorial which discusses the specific issues of having multiple NPCs act_busy, and I can't figure it out myself by looking at the example here:
https://developer.valvesoftware.com/...ctbusy#Example



How do I properly make multiple NPCs act_busy?!