My only thought/concern is that if you simply add a line and run them in series, you're basically stealing pressure from each return. you may not get the effect you want...

It seems to me that if your goal is to prevent algae from growing near your steps, I'd either only add one extra return so that you're only splitting the water flow two ways (which should keep a fair amount of power for both returns), or add as many as you want, but also add diverter valves so that you can shut down returns in order to maintain some pressure.