You should have no problem keeping them warm for 6 or 7 hrs. Take both briskets, wrap them in two layers of heavy foil, get an ice chest big enough to hold both briskets with some extra room. Line the bottom of the ice chest with an old towel, wrap each brisket in an old towel, place first brisket in, layer another towel, then place second brisket in, fill rest of the space with more old towels. Put the lid on and don't open it! They will still be almost to hot to handle when you pull them out of the cooler.
On a note of caution... since you know you are planning to cooler them for 6-7 hrs. I suggest you remove them from the smoker when the internal temp. on each brisket hits 185-190°. The carry over cooking will finishe them off in the cooler, if you take them all the way to 200° then cooler them they might get a tad mushy in texture.