I have a project where I'm using one 555 set to give a ping for 100ms at 400 ms intervals. I'm wanting to use another 555 to receive that ping and not change pin 3 status unless it fails to receive the ping from the first for 1.2s (to allow for 2 complete cycles with a little bit of a time delay/component variance). I'm having a mental block with how to do this and ensure reliability over a period of 10-15 minutes with no degradation over time. Any thoughts? Thanks in advance!

 

What you want is a missing pulse detector, usually implemented with a retriggerable monostable. Because there is no feedback from the output to any of the inputs, the standard 555 monostable circuit works this way. As long as it gets trigger signals faster than its own timeout period, the output never changes.

ak

 

Just expanding on AnalogKid's suggestion.
U1 inverts positive going pulses to give neg. inputs to pin 2.
U2 discharges C1 if input pulses are long enough, use ALt. if pulses need to stretched.