Wow, alot of misconceptions out there. I'm an insurance adjuster so I have a little experience in total losses.
First of all, don't mistake a stated amount policy for an agreed value. Basically, with a stated amount policy, you are setting your premium, not the value in the event of a total loss or theft. Secondly, there's not an insurance company out there that will tell you a value "in the event of a loss". Your agent may be telling you something, but don't confuse an agent with a claims adjuster that actually works for the insurance company. Most agents have no clue how the claims process works.
Laws are different in each state and each insurance company has different procedures, but I used to work for one of the "big 3" insurers. Their proceduces were basically the same from state to state. When a claim is made and the car is stolen or a total loss, the mileage, condition, and options are confirmed. This information is relayed to a company that determines the "actual cash value" of the vehicle. These companies keep databases of vehicles that have been sold or are for sale in your local market. So your value is based on what other similar cars are selling for.
I'd say maybe 30% of claimants will dispute the value of their vehicle. They are given an opportunity to do their own market survey (autotrader.com) to prove the value that they think is fair.
You never have to accept what your insurance company is offering, however you cannot sue. Check your policy (if you actually have a copy) and look for the "independent appraisal clause". It says that if you guys cannot agree, you each hire your own appraiser and what they agree to is binding.
The insurance companies aren't out to screw you out of any money. They see you as a number and want to settle your claim at the lowest ultimate cost and move on to the next one. A turbo Buick owner isn't going to have trouble getting a large settlement for a nice car. It's the dumb ass Regal owner that thinks their n/a or v8 is worth $10k that's going to have a wake-up call.
Oh, and you cannot have two primary policies on the same car. Information is shared daily with each and every insurance claims office and they will catch you in a second. Which ever policy you took out first likely immediately cancelled when you took out the second one due to contact clauses in your policy. You're paying premium for a policy that will likely deny any claim you present. Unless this is supplemental coverage, then you're okay.