Yes, a 2.0Amp 24VAC power supply will be ok. The OSPi and the RPi will together draw something around ~400mA (0.4A) under normal operation. The 24VAC input on the OSPi board has an inline fuse, which will blow if a fault…..such as a short circuit on an output, occurs. The OSPi will not draw more current from the power supply than it requires……just because your power supply is able to supply more current, it will not be used by the OSPi. The fuse will blow quickly, if a problem with the OSPi or RPi causes too much current to be drawn…..which protects both the OSPi and the power supply. There is also a secondary protection on the 5VDC output of OSPi, which is a polyfuse (see: http://en.wikipedia.org/wiki/Resettable_fuse for more details on how they work).
Hope That helps.