Why isn't there a procedure for dual-engine failure in twin-engine airplanes?
There are.  The Metroliner (SA227) has a dual flameout checklist.  It's possible that the company I flew for developed their own and had it approved, but we DID have a checklist for that.

I think they're rare on pistons twins because there are a very limited number of environmental factors that can cause double engine failures.  It's also possible to simply run out of gas.*  A turbine can ingest water, ice, dust, sand, snow, etc (no filters) that can contribute to a flameout.  The factor that causes one to quit will likely effect the other in a similar way.

I do know a pilot who flamed out both engines by using engine inlet heat on both engines at the same time.  The ice melted off the inlets at about the same time, both engines ingested the ice, and they quit.  He managed to relight both of them and landed uneventfully.  That likely would not happen in a piston twin.  The alt air doors would open and keep the engines running.

*If you find yourself thinking "I'm going to need that double engine failure checklist in about 20mins," you should probably seriously reflect upon the decisions you made throughout the flight.