watt voltage are you on turtle and these contrller boxes need to complete the circut to work if you have 2 trip boxes the 20 amp will probibaly have a boiler or simalar appliance on its circuit , follow it and see what is on its line (if you can ) the circuit can be bridged but the bridge wiil more likely have to have to be split to reduce the ampage from 20 to 15 if you post what is running on the 20 amp and your voltage my son will be here tomorow and i will confer with him and get the solution for you but we need voltage ok mate .
sorry about the spelling but took my meds and a bit drowsy (sorry) later turtle
PS DONT BRIDGE IT WITH A BIT OF WIRE AND A PLUG MATE YOU WILL GET INJURED THAT WIIL NOT BE SAFE try tripping each box and see what goes off then you will know what is on the 20 amp circuirt