I would just hook the 5.6 gpm washer up to a tank and use the 4gpm washer as a back up just my opinion
I was planning on two guys washing the house.
At least 225 gallon tank. Don't hook up a low water shut off. It will annoy the heck out of you if your machine cuts off every time you are low on water. You will know when the water is low. Use a decent pump and it won't mater if it runs a minute or two without water. I flow 2 8 gpm machines off of each tank. Make sure you plumb the machines seperately.
what i figure is u get better pressure hooking up directly to the house if u r running 6 gpm machines or less. especially if you have a small tank. if you have a large 200 gallon tank the weight of the water will create more pressure to the pump. the fuller the tank the more pressure u get comming out the bottom. if you are using 100 gallon or less you might no be getting enough water flowing fast enough to the pump. if you have a pressure gauge hook your pump up to the spiget then to the tank and see if there is a difference in pressure...did that make sense?
I would always pull from a tank. If your running a 6gpm I wouldnt take the chance of starving your machine from a resi supply. Also a belt driven machine will pull just fine no madder how much water is in the tank.