Origin has only one switch input, and it is hard-assigned to the hold (sustain) function. In my opinion this is an annoying limitation but it cannot be changed with the current firmware.
However I can see 2 workarounds:
1- use an expression pedal and assign it to the rotary speaker slow/fast parameter. To do this :
- go to Live/MIDI (bottom screen switch #8, and top screen switch #4)
- in the CC assignment list, navigates to CC11, select it,
- on the new popup got the MONO section and find there the Rotary Speaker, select it
- finally select slow/fast
2- if you own a programmable MIDI footswitch controller:
- assign one of your switches to CC66 or 67
- proceed as in solution 1 to assign the CC66 or 67 to Rotary Speaker - slow/fast
HTH