I think this would be problematic; you'd have to use 11x17 portrait, and,
unlike metric sizes, US sizes are not proportional when rotated. A better
approach would be to use "2 pages per sheet" in the Page Setup dialog
(Margins tab: Multiple pages), which gives you two 8.5x11 pages side by
side. You create the pages just as you would individual Letter-sized pages,
with page numbers, running heads, etc., but Word will print two on a sheet.