The Unseen Seer prestige class (CM) has the Divination Spell Power class feature, which reads:
At 3rd level, you gain a +1 bonus to your caster level when casting an arcane divination spell. This bonus improves to +2 at 6th level, and to +3 at 9th level.
This benefit comes at a cost: Your caster level for all other arcane spells is reduced by 1 at 3rd level. This reduction becomes 2 at 6th level and becomes 3 at 9th level. For example, a 4th-level rogue/1st-level sorcerer/6th-level unseen seer would have a caster level of 9th for her arcane divination spells, but only 5th for her nondivination arcane spells.
The Practiced Spellcaster feat (CA) reads:
Your caster level for the chosen spellcasting class increases by 4. This benefit can't increase your caster level to higher than your Hit Dice. However, even if you can't benefit from the full bonus immediately, if you later gain Hit Dice in levels of nonspellcasting classes, you might be able to apply the rest of the bonus. For example, a human 5th-level sorcerer/3rd-level fighter who selects this feat would increase his sorcerer caster level from 5th to 8th (since he has 8 Hit Dice). If he later gained a fighter level, he would gain the remainder of the bonus and his sorcerer caster level would become 9th (since he now has 9 Hit Dice). A character with two or more spellcasting classes (such as a bard/sorcerer or a ranger/druid) must choose which class gains the feat's effect. This feat does not affect your spells per day or spells known. It increases your caster level only, which would help you penetrate spell resistance and increase the duration and other effects of your spells.
If a character has both of these abilities (say, a Rogue 2/Wizard 3/Unseen Seer 8), how is their caster level calculated, for both divination and non-divination spells?
I can think of four possibilities:
1. Divination Spell Power applies first: Thus, the example character's caster level for divination spells is 13 (11 spellcasting levels, +2 for Divination Spell Power, +0 for Practiced Spellcaster because already at HD cap), and their caster level for non-divination spells is also 13 (11 spellcasting levels, -2 for Divination Spell Power, +4 for Practiced Spellcaster).
2. Practiced Spellcaster applies first: Thus, the example character's caster level for divination spells is 15 (11 spellcasting levels, +2 for Practiced Spellcaster to reach HD cap, +2 for Divination Spell Power), and their caster level for non-divination spells is 11 (11 spellcasting levels, +2 for Practiced Spellcaster to reach HD cap, -2 for Divination Spell Power).
3. The player decides the order in which they apply, but must choose the same order for both divination and non-divination spells: Thus, the player may choose between options 1 and 2.
4. The player may decide independently in each case what order to apply the bonuses: Thus, if they apply the bonuses in the most beneficial order in each case, their caster level for divination spells is 15 (per the reasoning in option 2), and their caster level for non-divination spells is 13 (per the reasoning in option 1).