I would use one page with two columns: English on the left, Spanish on the right. On your master page, set up left and right text frames that flow independently (are not linked to each other). This will let you flow all your English text into the left column, and Spanish into the right column. Create a paragraph style with "Keep Options" set to start paragraph in next frame, and "Export Tagging" with "Split Document (EPUB only)" checkmarked. You can use the same style for the first paragraph of each "page" in both the English and Spanish text.
Thank you @evansreb. Are you a freelancer-for-hire? This is the genesis of a project that I am working on right now.
I work full time but do some freelance design and composition.
Actually, upon reflection, I realize this won't help because the output is ePub rather than print. For a reflowable ePub, you would need to do something like put the English and Spanish in table columns so that they displayed next to each other. The way I describe above would display them side by side on screen but one after another in the ePub. I also realized that there is no need for two text flows, you could simply use one text flow with the English and Spanish texts following each other, using the page/ePub break paragraph style as described above. This lay them out as expected for a print version of the book. This book sounds like a good candidate for a fixed-layout ePub.
Our first support for EPUB3 Fixed Layout is now available with the 2014 (June 18th) update to Creative Cloud.
Download the trial and let us know how we can improve it.
Architect, InDesign Engineering