it really depends sa application mo.
natutunan ko rin to dito and thru experience...
if you use them individually, it wouldn't matter w/c one goes first.
but if you cascade them, you have 2 scenarios:
1) whatever you put first in the chain (from guitar), adds gain to the
second--->gain boosting
2) whatever you put after, adds volume and "color"--->volume boosting
on #2, volume is achieved when your setting is: 0 gain, max level (on the 2nd pedal). "color" meaning the sound is a combo of the 2 pedals.
hope this helps.