On the assumption you're using apache vhosting, it's as simple as settting the DocumentRoot to the same location for both. To use symlinks (as in Dan C's answer) ensure you have
Options +FollowSymLinks
Set in your httpdocs/.htaccess file, or your master httpd.conf as appropriate (.htaccess files are looked for and processed on each page load in that directory, so there's no need to restart apache, however they are slower than implementing the same code in httpd.conf as this file is only processed on server startup).
Here is an httpd.include to serve HTTPS data from the same location as HTTP:
<VirtualHost *:80>
ServerName test.com
ServerAlias www.test.com
UseCanonicalName Off
DocumentRoot /var/www/vhosts/test.com/httpdocs
CustomLog /var/www/vhosts/test.com/logs/access_log common
ErrorLog /var/www/vhosts/test.com/logs/error_log
<IfModule mod_ssl.c>
SSLEngine off
</IfModule>
Include /var/www/vhosts/test.com/conf/vhost.conf
</VirtualHost>
<VirtualHost *:443>
ServerName test.com
ServerAlias www.test.com
UseCanonicalName Off
DocumentRoot /var/www/vhosts/test.com/httpdocs
CustomLog /var/www/vhosts/test.com/logs/access_log common
ErrorLog /var/www/vhosts/test.com/logs/error_log
<IfModule mod_ssl.c>
SSLEngine on
SSLVerifyClient none
# example ssl certs
SSLCertificateFile /usr/local/apache/certs/my.ca.pem
SSLCertificateKeyFile /usr/local/apache/certs/my.server.key.pem
</IfModule>
Include /var/www/vhosts/test.com/conf/vhost_ssl.conf
</VirtualHost>
Useful links from O'Reilly's Step by Step: Configuring SSL Under Apache and the Apache SSL Page (verbose!)
Edit:
You should be able to access you conf/httpd.include file over FTP (it may be in a different directory, but it will be in the vhost's subdirectory of /var/www/vhosts/test.com/ or similar - you can find the vhost directory in your main httpd.conf).
You should also be able to access .htaccess files - they are linux hidden files, so you may need to check your FTP client's documentation. Alternatively, try creating a .htaccess file in your vhost's httpdocs/ directory - if you can't the file may already be present.
Last gasp: contact your hosting provider!