@asunw: I ask myself the same thing. I work in Fire Protection, and the higher ups at my company asked me to take it even though I think my mechanical PE (therm/fluids) would definitely cover fire sprinkler work. If I get the FPE then my expertise is less likely to be questioned. Also, there are some government jobs that require the FPE. And looking down the road, as far as jobs go, FPE is rarer and if I ever wanted to work in an A&E firm it might get be a good selling point that most don't have. If I get it, I'll be the only one at my work who is a double PE. But, I don't get another stamp or anything.
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?