General :: .htaccess RewriteRule Map URL To Another

I've developed a web program and let say the main url is [URL]

I will be distributing different URLs for different customers and each customer will have their down database. Let say the URLs will be:

[URL]

because i be using single instance for my program, both this URLs will be pointing to the base code at http://domain.com. depending on whether is customer1 or customer2, the program configuration will extract from the respective database.

my question is, how do i use mod_rewrite to map the different urls to the main url with transparency. meaning, although each customer will see domain.com/customer1, this is actually pointing to the main base code at domain.com which has a config file to pick out the respective database.

I've managed to do it with alias in httpd.conf. e.g:

alias /user1 /var/www/html/myproject
alias /user2 /var/www/html/myproject

however, if i were to do this, i would have to restart apache every time i've got a new user. so i've decided to use .htaccess instead but realised that alias is not availble for it.

I've tried with rewriterule but with no success.