It's not clear what you have now, but my guess is you are trying to print two letter-size pages to each side of a tabloid sheet.
The easiest way to do this if there is more than one printed sheet per document involved is to set up your file as letter, portrait, facing pages. Do your designing in Reader's spreads.
When it's time to print (and it sounds like you are printing in house, so these instructions are for doing so) you have two options: Export to PDF as single pages (which is what a commercial printer will also want) then use Acrobat's booklet feature to send to the your printer, or use File > Print Booklet... from ID and make sure you click the setup buttons to get all the way into the print driver to set the paper size to Tabloid, landscape, and be sure to check the box to print blank pages and set the page flip to the short edge.
The first method is probably simpler to execute, and is less prone to problems with incompatible drivers (ID is a bit notorious for not playing nice with some printer drivers).
Correct, two facing pages on the sheet. If that's what you mean. Like a magazine. But when I print, one design takes up the ENTIRE 11X17 sheet and it doesn't split in half.
Thank you for the fast reply!
So I changed it to what you said and it worked (thank you, you saved my life x) )
But now when I try to print it's cutting off quite a large chunk of the page. I have a gutter set for 3p0 and I've defined the page as 11x17, but it's not working in my favor.
Also, when I'm printing the cover, it leaves a entire border of white space.
I'm sorry for the troubles - I don't mean to be so clueless. I've designed with ID before, but I've never had to print such a large project.
I'm still not sure what you have for setup. Where is this gutter? Can you show us a screen shot?
White border would be normal on most desktop printers -- they mostly cannot print edge-to-edge. To get full bleed you'd need to print on over-size stock (12 x 18, if the printer will handle it) and trim.
Also, be sure the page position is set to Center if using Print Booklet.