It seems to me you would only need to add one extra hole in the bottom of your overflow for a total of three - one for the flow to the sump, one for the return from the sump to the tank and one for the return from the sump to the swirls. You might have to make the hole for the flow TO the sump larger. You must ensure you have enough capaciy for return flows (by gravity) to compensate for two pumps from the sump.
Not sure what type of pump you are using (external?) but I guess you would need an extra hole in your sump for the second pump. Is it possible for you to have just one pump and tee off its dicharge to the two systems?? Another question, since I must admit I'm not sure what sea swirls are, but can you eliminate the main (original) return and run all the return water through the swirls??
Assuming you want separate systems, it seems you would also need two additional holes on the top sides of your overflow chamber for the return lines to the swirls. This means there would have to be a tee somewhere in the plumbing inside the overflow chamber. If you do this, then you could run each line inside the tank along the top/back face. Now - how do you anchor this piping/tubing?? If your tank is acrylic with the edges on the top, I'm sure you could come up with some type of clip.
Hope this is of some help!!