Word – Microsoft word: Text outside page border

IF the excessive 'border' is on the left of one portrait copy and on the right of the other then reducing any gutter setting (Page Layout, Page Setup, Margins) may help - if it is not already 0! Otherwise Reduce Outside and Inside to suit.

If there is too much 'border' at the top and bottom reduce Left and Right Margin values.

With all settings at zero I doubt there is any way to reduce borders further (one at least is likely to be required by your printer).

However, you may be better off with Book fold rather than 2 pages per sheet. Gutter there controls the space between the two images (presumably you would want that set to about 0.4") Top and Bottom then rather more logically control the margins above and below the portrait copies and Outside the left and right borders, as a pair (Inside is like Gutter for Book fold).

All printers have a "printable area", how wide the margins to the true paper are depends on the make/model (and the paper size, obviously). Some printers are able to print right up to the paper border, but AFAIK that is rare. Printing on "larger than A5 paper" on a printer than handles up to A5 won't get you anything.