The Summon Evil Monster feat from Champions of Corruption states that "If you possess the Sacred Summonsfeat, you can apply it to a creature on this list whose alignment (as opposed to its subtype or subtypes) matches your aura."
Maybe I am just to tired, but how does these feats interact, and benefit having both.
Both Limit summons, but can work together. And both make casting the spell one action.
What is the beenefit of having both?
The Summon Evil Monster feat has a constraint:
The summoned creature appears as normal for the spell but can’t act until next your next turn. It is not flat-footed, however, and it can make attacks of opportunity as normal.
By contrast, using Sacred Summons, the thing you summon can act immediately. Sacred Summons basically allows you to summon anything on the Summon Evil Monster list (provided its alignment - and not its subtype, which is another advantage of having both feats - matches your aura) and have it act immediately it arrives, instead of waiting until your next turn.
The interaction consists in the fact that you can apply Sacred Summons to a larger list of creatures and also that if you apply Sacred Summons to a monster which matches your aura, you can summon it as a standard action and get it attack the same turn in which it was summoned. So, Summon Evil Monster gives you an expandend summon list to which you can apply your Sacred Summons feat (supposing your alignment is evil). If you don't have Sacred Summons, you can summon those evil monsters but they're ready to act only in the next round; if you instead have the Sacred Summons feat, they attack in the same turn in which they're summoned. I think it's a sort of overlapping. If you summon a creature whose alignment doesn't match yours, you can still summon it as a standard action, though it can't attack for that turn.
Sacred Summons usually requires you to summon a creature with alignment subtypes matching your Aura. The combo effect allows you to summon a creature with a matching alignment.