I work in an electrical/instrumentation design group. I took the instrumentation test in October, but if I didn't pass, I'll take the electrical power test in April. That's only because the I&C test is only offered once a year.
The lady at the TN board essentially told me I'd be crazy to take the electrical test if I pass the instrumentation test. Since they make no distinction, and as you say, you don't get two licenses, I guess it would only be bragging rights. Would you sign you name John Smith, PE PE?
People who work solely in fire protection want to see an FPE not just a mechanical engineer. The Fire Protection exam is completely different from the mechanical. A few exam topics like pipe flow and pumps are interchangeable. But, most others, like means of egress, building construction, life safety, and fire protection analysis are discipline specific. It's not just about bragging rights. It's two separate fields of engineering and by passing both I prove that I am competent in both as an engineer. I would sign my name most likely,
Jane Smith, PE
Mechanical/Fire Protection Engineer
My firm is a mechanical consulating firm in west TN (about 25% of our work is fire protection) and i have never had anyone ask if we have a fire protection engineer. Do you work in a specialty field? I have my PE but have never considered the FPE because of the reasons already mentioned here.
All I can say is, this is how it was presented to me, my company works solely in Fire Protection, and I was asked by the VP to take it, so I did. I do have a colleague and he is a double PE (mechanical/fire protection). So, in my limited experience (I've only worked at this company since getting my engineering degree) it didn't seem so odd. So I took the test.