How can I set different sensitivities for two mice at the same time?

You can use xinput to set the sensitivities/accels for the mice.

$ xinput list
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ Logitech USB-PS/2 Optical Mouse           id=8    [slave  pointer  (2)]
⎜   ↳ Microsoft Microsoft® Nano Transceiver v2.0    id=10   [slave  pointer  (2)]
⎜   ↳ Microsoft Microsoft® Nano Transceiver v2.0    id=11   [slave  pointer  (2)]
⎜   ↳ Macintosh mouse button emulation          id=13   [slave  pointer  (2)]
⎣ Virtual core keyboard                     id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Power Button                              id=7    [slave  keyboard (3)]
    ↳ Microsoft Microsoft® Nano Transceiver v2.0    id=9    [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=12   [slave  keyboard (3)]

From that output take the numeric id of the mice. I'll provide an example for my Logitech mouse. For multiple mice rinse and repeat.

So for my logitech I will use xinput get-feedbacks <device name> and xinput set-ptr-feedback <device name> <threshold> <num> <denom>

$ xinput get-feedbacks 8
1 feedback class
PtrFeedbackClass id=0
    accelNum is 3
    accelDenom is 10
    threshold is 4

The accel is set as a fraction so you need to set the nom and denom for it:

$ xinput set-ptr-feedback 8 4 3 1
$ xinput get-feedbacks 8
1 feedback class
PtrFeedbackClass id=0
    accelNum is 3
    accelDenom is 1
    threshold is 4