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%.
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
Damage or 77.792% Damage Taken