Board Thread:Dark Souls II/@comment-25071275-20150426020545/@comment-4511886-20150426122313

Bosses, and some enemies, gain defence equal to 25% damage reduction per summon (two summons means you only do half of your normal damage) which is how they balanced the bosses for co-op. In Dark Souls, they boosted the boss' HP by 50%, if I remember correctly, meaning that the difference wasn't quite as obvious, but still noticable.

For this reason, getting a second summon can actually make it harder, as the defence boost will greatly reduce the speed of taking the boss down.