Well, I realize this is a situation where size matters since you pay OSH Park by the sq. inch. The additional cost for components is 25 cents or so.
Add a 10k-10k divider that generates a 2nd trigger signal that is 1/2 the amplitude of the original trigger. Add a 100K-0.1uF combo to set the blink pulse duration of the 2nd timer. Add a resistor-LED for the 2nd LED.
The idea is as follows. You have two trigger voltages, one being 1/2 of the other. The 123 chip will trigger when the voltage at the "B" input is about 2.5V. We'll call this the full-scale trigger for a good TIU output. The other trigger would then be about double that or 5V which of course will trigger the other half of the 123.
But if the TIU level drops, the lower trigger will fall 2.4V, 2.3V, 2.2V, etc. while the higher trigger will fall to 4.8V, 4.6V, 4.4V, etc. So in this case only one LED will blink.
So rather than a go, no-go the creeping feature syndrome makes this a go , maybe, no-go. 
