Quite honestly, I think that's a tricky situation to manage effectively. The horses should be seen daily to ensure there's water, food and they're not injured as well as checking the condition of the fence. It's also very easy for a couple of horses to disappear, whether by their own design or with help, in that period of time.
As far as feed is concerned, the obvious response is to put out a round bale that would likely last more than a week. For water, you would need (assuming there's not a stream, etc.) multiple troughs (or one really big one) filled up to match what they would drink in six days. I don't know where you are but you may still be left with needing a solution to maintain the quality of the water during that time if it "spoils" because of heat.