Separate them. Don't reward their bickering by giving them one-on-one attention with separate parents, or a nice play date with other friends. (Although I do think it's important that they have separate friends.) An incentive jar is a fine thing but don't make it so difficult to enforce and monitor that it makes YOU overwhelmed, especially when you aren't home. And don't let them think that they can be mean and nasty and then just make a few nice comments within earshot of you to get marbles. The proble with rewarding the kid with the most marbles is that it sets up a competition, and that's already the problem - they compete instead of cooperate. And I'm not sure what you do with the other kid when you take the "winner" to the movies or the pool. The point is to get some cooperation and some peace and quiet through cooperation.
I would tell them you don't want to hear it, that they may think their nastiness is getting them somewhere or that you're going to take sides, but unless someone is bleeding, has a broken bone protruding, or is on fire, you don't want to know about it. They are old enough to shut their mouths, and people have to learn to deal with others they spend the day with in school, camp, families.
Try not to snap - I know it's so hard - but it reinforces the idea that people who are irritated by others should be snapping. Which is what they are doing with each other. Try to do the opposite - send them outside or to their separate rooms or corners. (like boxers in the fight ring, they have to separate) Do not engage them at this time - leave them alone. If they have any electronics or computers/iPods in their rooms, take those away. Set a timer for 15 minutes. Second offense 25 minutes. Third offense, 35 minutes.
You can try an incentive jar, but another option is a chore jar. If they have time to nag and whine and argue and demean, they have time and energy to contribute to the welfare of the family. Take a bunch of cardboard strips and write chores on them, then put those in a jar or paper cup. They have to pull out a job and complete that chore. Alone. No arguing or they take 2. Chores for 5 and 6 year olds can include sorting laundry (lights and darks) and putting a load in the washer (print instructions about how much detergent and the water settings and tape it to the washer front so you don't have to engage in a discussion, which gives them attention), unloading the dishwasher, setting the table for the next meal, taking out the trash and recycling, cleaning the cat litter box, running a vacuum or a swiffer on the kitchen floor, dusting, matching up clean laundry (socks, underwear, towels are easy and wrinkles won't matter). Of course, picking up toys and dusting their own rooms should be on the list, reorganizing their sock drawers and their shoes in the closet, cleaning out the hall closet and organizing boots and other off-season items, pulling weeds in the garden, sweeping the garage are all things that can be done by a kid this age and which don't have to be perfect and there's little or no chance of anything valuable being broken. Once the job is done, the cardboard strip goes in another jar or cup until you decide that the job needs doing again, in which case it goes back into the regular jar.
They'll find out that they can earn the good times by not participating in the nonsense. And if they fight, they BOTH get jobs - it doesn't matter who started it. That sucks you into the debate and makes you the referee. If you SEE someone do something to the other, then the offender gets the job. The ones who gripe at you and complain over and over get an additional job. Then you LEAVE the area and do not hear the fallout.
If they don't get an audience, they'll settle down.
Good luck. I know how frustrating it can be and how easy it is to just explode and give them a piece of your mind!