There's a bug affecting all magento 1.9 versions, that prevents users from applying multiple coupons to their order--even if "Stop further rules processing" is NOT selected.
Basically, a customer can only apply a single coupon to their order--no stacking of coupons works in Magento 1.9.
Here's the fix, you'll need modman and generate-modman installed:
cd /your/magento/installation cd .modman git clone https://github.com/husseycoding/cartrulefix.git cd cartrulefix generate-modman cd ../../ modman deploy cartrulefix rm -rf var/cache/* var/session/* var/locks/* var/full_page_cache/* tmp/* && rm -rf downloader/pearlib/cache/* downloader/pearlib/download/* && service httpd restart && service memcached restart
That should do it.