Surely it would be better to match two class 20s that each have a decoder. This way you could use them independantly, it would make it easier for cleaning and maintenance and should one of the 20s fail, as they did from time to time in real life, you would still have one in operation.
But should still go ahead with permanently coupling your locos, you would need to know and understand what loading you can put on your decoder. I,m referring to amperage output and current draw from two motors.
I occasionally run double headers but they are matched under the same address (usually the same model from the same manufacturer). Each loco will be fine tuned so that they are not working against each other when paired. Although the two loco have the same motors, it doesn't mean they are forced to perform the same.
The Lenz system that I operate, allows for double heading or Multi Unit/Consist, where each loco retains its individual address but once coupled can run under a new address within MU/Consist function and operate as one unit. Very useful for banking situations.