Mekabar said:
No. The +1 caster level only applies to spells on the given domain list.
That is not correct. It applies to all spells of the given alignment type. It has nothing to do with what spells are on your domain list. If you have anything to show otherwise, lets see it. As it reads, you get a +1 caster level to spells with the particular descriptor, not just your domain spells.
Summoning a lawful evil being doesn't make Summon Monster a spell of the law and evil domain.
While true, it's not relevant. The spell still has the lawful and evil descriptor if it is a summon monster spell of a lawful and evil creature, thus gaining the +1 caster level.
In my opinion, the +1 caster levels DO stack. There is no multiplication here, so those multiplication rules do not apply. In addition, it is NOT a specific modification to a d20 roll (not attack rolls, damage rolls, saving throws, etc...), though it can have an indirect effect on a d20 roll, so I do not think the stacking rule applies. Finally, even if the stacking rules do apply to a bonus to caster level, the "bonus" in this case is unnamed, and therefore the stacking rule does not apply. Any way you look at it, the two +1 caster levels appear to apply just fine, and your summon monster spells would in fact be +2 caster level if you had both alignment domains and summoned a monster that met both alignment requirements.
Why this is a big deal to some I have no idea. So you get your summoned monsters for 2 more rounds, for having chosen some relatively mediocre domains. That doesn't in any way sound overpowered to me.