Quick update now we are all melting....
Ehiem 2213 paired with 2 x 60cm tanks, water flow is too slow. Temp difference between tanks is less than 0.1 degree. However on hot days the tanks slowly creep up in temp to 25 degrees, even setting the chiller to 21 degrees. Water has to travel to a max height of 210cm, the canister is sitting on the bottom shelf just a few inchs off the floor so I guess there just isn't enough grunt with flow loss via all the tube bends. Tried removing all the media,this made very little difference.
Replaced the Eheim with an inline Resun King 2FLV 1000l/hr, I run this externally with 2 x 12mm hose barb fitting to 1/2in bsp male adapters (package only comes with one of these other is a 19mm hose barb to 1/2in bsp). I set the chiller at 22 degrees and now both tanks sit at between 23 and 23.6 degrees. The delta between the 2 tanks varies from 0.1 to 0.3 of a degree.
The loss of filtering I compensate with Qanvee QS200 sponge filters packed with media and modded to run off an Aquael Mini Patfilter rather than an airpump. (Much quieter too).