Please explain why. The units have a completely variable input, from
zero to about 150 volts. The laws of a voltage divider circuit will
apply to two in series. To me, that means roughly the same voltage
across each one.
You have no way to ensure the voltage will divide equally across
the two MPPT's. That happens *only* if they always drew equal
currents, and such would a fiction soon exposed.
Then the greater voltage will be across one or the other MPPT,
it will suffer from escaped smoke, and when it does, the full
voltage will likely be across the remaining unit; which then
joins it in the "WAS a MPPT but is now a paperweight" catageory.