I have researched the forums a bit for information on these topics. I used the + advance function and still have not successfully located an answer. Kudos to anyone who finds a thread on this topic.
I have an Aasimar and used the blood of angels handbook. Replacing my spell like ability with DR 2/evil
My eidolon gains DR 1/evil for ever 2 points invested as a favored class ability replacement.
Would the DR stack for both granting DR 3/evil when fused or would the eidolon have DR 1/E until it was killed leaving and the synethist to have DR 2/E. The wording of the fused eidolon says "the two are now one creature." I know DR of the same type stacks but it is kind of a special case.
A took a 2 level dip in monk to gain Master of many styles, going with the Boar style tree. Reading a bit more I assume it's no, but while fused using the eidelons claw attacks, can you inflict bleed damage with 2 successful hits?
The summoning takes 1 minute. If say a surprised round of combat erupts during that 1 minute, who would take damage upon being hit. summoner or eidolon?
Can a synthesist walk around in fused form indefinitely? outside of role playing complications (you are a scary looking monster thing, towns folk may not like that). Do you eat, sleep etc? I imagine that the Synthesist needs to do those things but the Eidolon may not because its an outsider creature that is treated as a summoned creature. so could the eidolon continue to function while the synthesist sleeps?
Lastly, impossible hypothetical with the wording of the class but assume somehow the synthesist were to be killed while fused,without killing the eidolon; what would happen?
Here is the thought train that led up to this: since the synthesists has the mental stats, is that to imply that the eidolon does not have any mental stats of its own; and it would fall into a catatonic state (because it would have a 0 for its mental stats); Or based on the level 16 ability of split forms, would imply that the eidelon is able to function away from the synthesist could you in theory have a eidolon walking around with a corpse synthesist "inside" of it?
Answer you your first question
If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.
So your would only get your DR 2/Evil
1. I would think the summoner would take the hit, as the Eidolon is not fully in this world yet. If you are worried about it, play an elf, and take their racial favored class bonus.
2. You still need to eat/sleep, so when you sleep, your Eidolon is gone.
3. My understanding was that since you are 1 creature, if the summoner dies, the eidolon dies too, no matter if it is damage, death affect, old age, ect. And when the eidolon dies, it goes back to its home plane.
DR never stacks unless it is explicitly stated to. So, your DR/evil does not stack with your Synthesist Eidolon. You use whichever is higher when you are fused.
No you cannot use Boar Style with the eidolon's natural attacks. However, if you pick up the feat feral combat training this will allow it. However, be aware Feral Combat Training applies to only one type of natural attack. So it could apply your eidolon's claws, or bite, or slam, or whatver. But only one of them. Since you need to hit with at least two for boar style to kick in, choose wisely.
The eidolon isn't summoned until the 1 minute duration is complete, the summoner takes the damage.
He counts both as his original type and as an outsider for any effect related to type, whichever is worse for the synthesist.
As the synthesist still counts as his original type, which probably needs to sleep/eat etc, the fused synthesist still does as well.
Short of something I'm unaware, it is impossible for the synethesist summoner to die before the eidolon armor. In any event, if it somehow did the eidolon is sent back immediately to its home plane.