I have 2 chillers as well and I was able to get mine from just short of a full rolling boil to pitching temps in about 20 mins. I have a submersible fish tank pump that maxes out at 291gph (according to the label on the pump). I put that into my kitchen sink and filled it with ice and just a little water to get the pump started. Off the pump, I have 1/2" silicone tubing that goes to my first immersion chiller (3/8", 25ft, stainless steel) that's sitting in my brew kettle. From there, the water goes through 3/8" silicone tubing to my second chiller (3/8", 25ft, stainless steel) that's sitting in my bottling bucket that's also filled with ice. Off the 2nd chiller, it goes through 3/8" silicone tubing and empties back into the kitchen sink where the pump picks up the water and cycles it through all over again.
When I turned the pump on initially, I watched the temp drop right in front of my eyes from 210* (give or take) down to 190*, almost instantaneously. From there, it was a relatively steady decline until it leveled out right at 100 degrees at 15mins. By then, all my ice had melted since I wasn't as prepared as I thought I was. My tap water was only getting as cold as 65* and the water coming out of the 2nd chiller was still holding right at 95*-100*. I kept the pump running with cold tap water and after the final 5mins, I was able to pitch at 85*.
Next time I'm going to try using my 2nd chiller as a pre-chiller rather than a post-chiller just to see if it makes a difference. I'm trying to stay away from salting the water because I don't want my pump to ingest the salt and tear it up. Maybe I can salt the water that the chiller is in (either pre- or post- ) and next time I'll have more ice and hopefully I can get it down to pitching temps right at 15mins.