Applying different master pages to odd and even pages which aren't in spreads

I need to print 500 A6 flyers which are double sided with different artwork on the front and back.

Therefore I am trying to set up my document with 1000 single pages with odd numbered pages being the front and even numbered pages being the back so they are ready to be imposed for print.  (These can't be set up as spreads because of the bleed which is required on both)

My question is how I can apply the two different master pages to the odd and even pages without having to go through the document and apply them all individually 1000 times!

Any help on this would be great!