It did occur to me that it could work this way, but given they are separate stat lines they would theoretically be multiplied. But to be sure I tested, and ultimately I found within a rounding error of 1 damage, that they do work in the way I described.
In my test I have, 40739 PDEF, 2497 Medium Offsetting, and 979 Leather Offsetting, and no other damage reductions. I am level 450, against a level 398 Infro Crabil which has 3852 PATK, and I tested on his 100% skill factor skill.
My calculated damage received is: 136
My in game damage received is: 137
Calculated Medium Offsetting: 2497/(398*30)*100 = 0.2091
Calculated Leather Offsetting: 979/(398*30)*100 = 0.0819
Calculated Total Damage Reduction: (1-0.2091)*(1-0.0819) = 0.72
Calculated Damage Received: (3852*0.72)(min(1,log10((3852/(40739+1))^0.9+1))) = 136

If they stacked additively:
Calculated Total Damage Reduction: 1-(0.0819+0.2091) = 0.7
Calculated Damage Received: (3852*0.7)(min(1,log10((3852/(40739+1))^0.9+1))) = 132
Summary: From my testing different Offsettings are multiplied together.