It depends on the Verizon store you go to. If you go to a corporate store, they may let you do that with one phone. I'm not crazy about those stores, I had to take a number and wait in line for close to an hour just to return a car charger for my phone. Your best bet is to try and find an independent agent that deals Verizon, they may be more willing to do something like that.
Either way, and I can't stress this enough,
MAKE SURE that you find out not only how long you have to try out the phone, but more importantly,
how many minutes they will allot you on the phone you're trying out. Most, probably all, places only give you a certain limit besides the 14 or 30 days. Usually it's a certain amount of minutes, a decent amount, to make some calls, etc., to see how the phone and service work for you. The reason they do this is because after a certain amount of minutes, if they have to sell the phone to someone else, it won't be considered a new phone if there are alot of minutes 'racked up' on it.
If you're trying to compare phones, here are some good sites to check out, :
Phone Scoop,
Cell Phone Reviews and News - Welcome to Mobiledia and
CNET - Technology product reviews, price comparisons, tech video, and more. They have user reviews and staff reviews which are both helpful and also comparison charts where you can check the phones you like and look at them altogether with all the features listed. They also have rating systems on individual features and the phones overall.
This might help some of your decision making and take alot of the guess work out of it so when you do go into the store, you'll be very well informed and the salesman will not try and pull one over on you.
Good luck!