MPPT charge controllers in series to handle high voltages
If the controllers were put in series, it would create issues for the MPPT algorithm because the algorithm calculates how much power it is drawing from the solar panel and the current from each MPPT controller would be the same (because they are in series) so each controller would calculate an incorrect power from the load.
In addition not each controller is created equal, so placing them in series will cause each one to draw a little more or little less power.
Another problem is the controllers probably don't have isolated grounds on the inputs and this could create problems for the electronics as they would not be referenced to ground.
Probably not a good idea.