Spell Resistance

Spell Resistance stacks the same way Armor does, except it works against Spell Damage instead of Physical.
All heroes have a base of 20% Spell Resistance [unknown base P.R., is probably 20%], meaning that you don't take as much damage as your creeps do.
So two 10% Spell Resistance items is inferior to one 20% Spell Resistance item by about 1%.
1 * 0.9 * 0.9 => 0.81
1 * 0.8 => 0.8
It's not that hard if you understand the above formulae (Attack Speed and Time Scale).
E.g. Incoming Damage * Base Hero Resistance * Nullifier * Void Steppers * Raynor's Lv4 Marked For Death
1.00 y * 0.8 * 0.85 * 0.88 * 1.30 => 0.77792y Damage or 77.792% Damage Taken