Bit complicated to explain..
See the repro mission it has all the triggers laid out already.
But to put it simply:
> make a mission with 3 units, name these p1, p2, p3 respectively
> make a trigger with the condition:
(!alive p2 || isNil "p2") && (!alive p3 || isNil "p3");
> set the activation on that trigger to hint "blah blah"; or whatever
> play the mission and disable one of the AI, slotting yourself in the first
> kill either p2 or p3, whichever is there
> observe the hint (or lack thereof)