Hi hoyess,
I'm in the same predicament. There is an overflow valve in the back of mine. I'm going to try to rig it up with pvc plumbing pieces to flow from one into the next. And yes, I'd presume you'd have to place the second one downhill from the first one. Gotta get working on this one, as we have a tropical depression headed this way tomorrow.
Good luck,
merj

I did add a hose to one of my rain barrels which happens to be at the end of one of my beds. The hose was great to let the water out in a slow steady stream. Soaker worked great. My barrels do sit up off of the ground a bit so the flow is normally good. Awesome water source ( not that I've seen much of it for too long but)- work around the sight of it and put it closer to the house I say!