A double spread would be 8.5 x 11 per page
And set it up as facing pages.
Page 1 is right hand page.
Page 2 is left - Page 3 is right
So you're Double Page Spread (DPS) needs to be setup on odd/even page
As facing pages.
You'll need to account for the spine width in the calculation of your margins for the inside margins.
For example - your margins might be 20mm all around.
But in the margins towards the spines - I'd make this 30mm - this indicates safe type area where you want information to stay away.
This accommodates for the fold in the page
