But you can use a "jack shaft" type solution. That allows you to double gear down the speed. Using two speed controllers in a row doesn't seem to me like it would work. Slowing down the motor past a certain RPM isn't a good idea either, if the motor has a cooling fan built into it. Once you get to a certain point the fan won't keep the motor cool. In case your not familiar with the term "jack shaft" -
Again just my way of doing things like this, the way I was taught back when horses were still powering things(a joke). You need to figure what SFPM(surface feet per minute) or inches per minute is needed. Then you calculate the gearing to get that from your motor RPM and roller diameter. Then with a variable speed motor you would try to get that SFPM with the motor running in the mid-range of the controller. Doing that will allow the ability to speed up when needed or slow down when needed. That is how the ready made ones are built/designed, to give the most flexibility of use.