How do they interact? Do they boost both bloodlines? Neither? Take for instance, a 16th level Crossblooded Arcane and Dragon blooded sorcerer. Does he get Arcane Apotheosis and Power of Wyrms? Or does he have to choose? Would two items each granting the benefit to one bloodline let him get both, or either?
Interested in a rulig and in the opinions of other posters.
Crossblooded doesn't get 2 bloodlines, they get 1 bloodline that is a combination of the two. Each time they get a power (1st, 3rd, 9th, 15th, and 20th lvl) they choose the power from one bloodline OR the other. So the Robes does indeed give its full bonus, to whichever power you end up choosing. There is no way to get the 20th level bloodline power from more than one bloodline. (unfortunately :P)
The Robes should still let you get said ability 4 levels earlier though.