Again I am not familiar with the policies in Europe or other countries and I am not giving any advice on anyone's insurance needs here. (Sorry to be repetitive but I have to make that clear or I could get in some trouble).
and I assume some other things.
Yup all of your personal property that is not attached to the dwelling should be covered under the policy. Though the more expensive items might need to be scheduled under the policy to be covered.
My excess is lower because I have a student policy, I'm fairly sure it's lower than the cost of either guitar individually, let alone both combined. Also, remember that I'm using GBP, not USD. £400 is about $640.
If the deductible is less then the guitar then ya it might be worth filing a claim for it alone but that all depends on how much the premiums would raise from that claim to. But I'll stop now ;p we are getting a bit off topic lol
Not sure why someone would insure something like a guitar, especially both. Only insure what you can't afford to lose.
For example, I would insure my house because if I loose it, I can't afford a second mortgage and I'll go broke. Contents are insured to an extent because if I loose all of my furniture, I can't afford to buy more and it is a necessity. I have 3rd party insurance for my car because if I get sued I can't afford to pay someone's hospital bills.
I don't have my life insured because my wife has a job and can pay off the mortgage without me. I don't have insurance for my 5 guitars because, while it would suck to loose them, I can afford to loose them because they are not a necessity for survival. I'd just have to wait for a while and save up the money to buy a new one.
Now, if your job is playing guitar, then losing that would mean losing your livelihood and you can't afford that. Therefore in that case I would certainly insure one guitar (maybe more if my income requires more).
Insurance just means that you are paying for the ability to immediately replace what was lost.