I have a question. I am in the process of building a rainwater collection system and I want to know if the water will flow like I am thinking it will.

I have (2) 55 gallon barrels located in my garage which I plan to use as holding tanks. These are connected together on the bottom to maintain level between the two barrels. They are on blocks at floor level. I located them inside for 4 reasons. 1) out of the sunlight to cut down on algae growth, 2) close in proximity to other components of the system, 3) to mitigate freezing concerns (I live in VA.), and 4) for visual reasons (my longest gutter is on the front of the house and I didn't want 2 barrels in the front yard)

Now, I am thinking of using a 4" diameter PVC collection pipe in place of the down pipe at the gutter drop, with a 2" PVC pipe serving as the feed pipe to the bottom of the barrels. So I will in effect have a 2" horizontal pipe (approx. 18' long, running at ground level) with a 4" vertical pipe at one end (approx 5' tall) and (2) 55 gallon barrels at the other end. My question is: when the 4" dia. vertical pipe on one end fills with water, the water will run into the horizontal pipe and begin to fill the barrels, but will the barrels maintain the same level as in the 4" dia. vertical pipe, assuming input flow rate isn't too much to overwhelm the 2" horizontal pipe?

To get a visual on this, imagine a local collection reservoir at the gutter (the 4" dia vertical pipe) and a remote storage location (the barrels approx. 18' away) connected together with 2" pipe at the bottom. As the vertical pipe fills up, will the water level in the barrels rise at the same level? Thank you