In general, hull tanking is inferior to shield tanking, see Shield Tanking for an explanation.
When a weapon projectile hits hull, the actual applied damage is subject to a piercing check:
If the projectile’s piercing is higher than the hull hardness, full damage.
If the projectile’s piercing is smaller, then the damage dealt is multiplied by Piercing/Hardness(reduced damage).
Most projectile types can breach the hull. See Elite: Dangerous Armor and Module Damage Mechanics by CMDR Arcvale for a detailed documentation of related mechanics.
Module Reinforcement Packages reduce module damage by applying it to the MRPs integrity itself.
An MRP protects internal modules with a reduction of 60%, and external modules with 30%. When an MRP reaches 0% integrity, the reduction is lost.
Multiple MRPs stack by multiplying the resulting rest of the damage applied to the original module. Running two MRPs would result in a protection of 84% (1.0 - 0.4 * 0.4) for internal modules, and 51% for external modules. All reduced damage is applied to the MRP that is highest in the list, except that Military Slots are used after all Optionals.
Therefore, it's a good idea to have a larger MRP in a non-military slot, then 1 or 2 smaller MRPs.