Not sure what you are gaining by having a back up heater. I have all of mine on one circuit and set at >77 off and <77 on. If you were insistent on a secondary heater I would make the biggest heater the primary and back ups at only .1 or .2 less then desired temp. The smaller heater as primary would be on longer trying to keep up. Also, not sure why people want such a swing in temp. eg <77 then on and <77.5 then off. IMO this is stressful for the livestock. <60 then off in not sure what this is for. So, IMHO if you want two separated heaters.
1) biggest heater as primary
2) program back up heater to come on at .1 less then big one.
3) Set temp > and < at same.
4) no defer or min or <60 in programming.