If a player has a shield with +5 defending shield spikes on top of a +5 (two sets of enchantments and two different sets of costs) shield and they decide to attack with shield and use all of the shields spikes enhancement on defense could they still use the Shields Enhancement bonus (not the spikes) to attack due to shield master feat?
Yes it stacks, the defending bonus is an enhancement bonus to AC, plus the rules specifically state this stacks with other enhancement bonuses as well.
Defending: A defending weapon allows the wielder to transfer some or all of the weapon's enhancement bonus to his AC as a bonus that stacks with all others. As a free action, the wielder chooses how to allocate the weapon's enhancement bonus at the start of his turn before using the weapon, and the bonus to AC lasts until his next turn.
So a defending spiked shield is a VERY nice AC item. Was thinking about doing the same as well.
Expensive though.
150gp masterwork shield
310gp masterwork shield spikes
25,000 +5 shield
72,000 +5 Defending spikes
= 97,460gp plus the cost of the shield and special materials