That is where it could get tricky. It might just produce all whites. It could also be that there is gene in there that causes a fatal or weak bird with the 2x white pairing like how you dont breed 2 frizzles together. Would need to check with people who have actually bred them and document. No...
I got interested and goolged the white emus. It looks to me that whites are a leusistic coloring or a blue eyed white. So that would make sense that you would need 2 blondes to get a white.