According to Aqua Euro's link, you need 160-290 GPH going thru your chiller.
http://www.aquaeurousa.com/Chillers.htm
If Rio is your only choice, it seems that the 3100 or greater would be right. With such a small sump, I imagine that you need a very narrow pump and that's y you're using a Rio? If you can, a Water Blaster 5000 would be a better option. At 10' of head pressure, it will produce 332gph and you need around 290gph according to Aqua Euro's instructions.
Typically the range on a chiller relates to whether or not your are near the minimum or maximum of that chiller's capability. If you are near the chiller's maximum capacity then you will use the slower rate of flow that it calls for because your chiller needs more contact time with the water to be able to cool it down. If the tank is small and you are near the chiller's minimum capacity then you will use the higher flow rate. So being that you only about about 15 gallons in total water volume you should be shooting for the 290gph flow rate that Aqua Euro calls for. If you were maxed out at 50gph then you'd shoot for the 160gph flowrate that they call for.
With the options that you're considering, adding a "Y" fitting may not be an option because the chiller would need full flow of your pump and being that they're on the smaller side, they may not have enough power to push thru the chiller plus the rise of the return line into the tank.
Your overflow maxes out at 200gph tho. Most overflow boxes can be pushed significantly but that is at each aquariast's own risk. I've pushed all of my own HOB overflows by double what they're supposed to handle BUT i've never used a nano overflow box so I have no idea how it will handle it if it all. So if you go with a larger pump that can handle approx 10' of head loss than it would be best to add a ballvalve to it just in case to slow it down if needed.
If your chiller needs over 200gph (almost 300) and your overflow can handle around 200gph and on top of that you want to split your returns than you either won't have enough going thru your chiller, possibly nothing significant going thru your other return (if you split it) or too much flow for your overflow to handle.
If I were dead set on having 2 returns, then I'd at least attempt it and see how it goes knowing that I may have to shut one down if it doesn't pan out. You won't know for sure until you try and you can always compensate with your powerheads.