Add these three commands in the appropriate .texi file after @end titlepage.

@headings double
@setchapternewpage odd

The first command selects A4 paper with better margins than the @afourpaper command. The second command turns on page headers suitable for two-sided printing (duplex printing). The third command starts the next chapter on an odd page number.

If you store those commands in the file a4.texi, you can simply say @include a4.texi in the other .texi files.

Remember to run texindex filename.?? between each run of pdftex filename.texi.

Here’s a patch for bashref.texi, the Bash manual.

--- bashref.texi.orig   2014-02-22 19:20:36.000000000 +0100
+++ bashref.texi        2016-04-17 00:22:26.141434000 +0200
@@ -51,6 +51,8 @@
 @end titlepage
+@include a4.texi