You will have to add the following lines to the default.lua in the
C:\Program Files\Eagle Dynamics\DCS World\Mods\aircraft\Su-25T\Input\su-25T\joystick or/and ......su-25\joystick folders
--Alternate Flaps
{down=iCommandPlaneFlapsOff,up=iCommandPlaneFlaps,name='Flaps Up - Alternate',category='Systems'},
{down=iCommandPlaneFlapsOn,up=iCommandPlaneFlaps,name='Flaps Down - Alternate',category='Systems'},
In this way two new flaps commands, 'Flaps Up - Alternate' and 'Flaps Down - Alternate' will be created in the Systems Controls Menu. Then, bind BTN22/BTN23 only to those two new commands and bingo.
Good Luck