IMO it is very difficult to execute a simultaneous rest with the thumb and another finger either I, m, or a. I have not done it a single time. I find that it is natural for the thumb to play a free stroke and the finger come to rest on the neighboring string during simultaneous plucking. I don't recall any discussion about developing the technique of dual rest stroke. Perhaps in time I will learn to do it but right now it is limited to a mental exercise. It should obviously be a part of simultaneous plucking but it does not have to occur for that process to be successful.it was a challenge to play rest strokes with the thumb and i or m fingers simultaneously.
So there were several occasions when I didn't manage to pluck the strings at the same time.
I think some discussion about the methodology of these separate but related processes would be beneficial.