I merged the two codes, deleted the lines that seemed repetitive, but still got an Internal Server Error.
However, I guess the following two statements could be contradictory.

# Handle all other URIs using Zen Cart (its index.php)
RewriteRule .* /index.php [QSA,L]


and

# Handle all other URIs using Zen Cart (its index.php)
RewriteRule .* wedding_prom_dresses/index.php [QSA,L]

Or can both of them stay together in the same .htaccess file?