When you say shorter period, did you finance for less than five years? Five years is standard, but now, dealerships will let you finance a car for 7 or even 8 years.

You stated that you have negative equity now. That tells me that you didn't put any money down or you put minimal money down. However, if you financed the cars for less than five years, you won't be under water on the loan very long because paying a car off in under five years will create equity no later than two years into the loan.

If you have a $1,500 a month car payment at 6% interest for 4 years, that tells me that you purchased a car worth about $65,000. Again, you'll have some equity after two years under this scenario.

Ultimately, the proper decision depends upon how difficult it is making the car payments after taking into consideration all of your other expenses. If money is extremely tight or you are even racking up credit card debt, then the answer is simple: sell one or both of the cars and buy something much cheaper with a lower payment; you will likely have to do this via trade-in, which is not a good financial deal, but really may be your only option and just have to bite the bullet to get out of the debt situation.

If you can afford the payments for the next few years and you plan on keeping the cars long-term (beyond 100,000 miles) then simply keep making the payments until they are paid off. This is most likely the best route, economically. Keep in mind, the rule of thumb is to either trade in a car before 80,000 miles or drive it until it dies. I drive a 2012 Acura TL with 334,000 miles. I'm driving it until it dies.