I can find the option to have a couple versions in custombuild 2.0, but not 1.2. Not having much luck with my searches. Maybe I'm just too tired. I ran in to a case where I have a few sites that simply won't work with 5.3+, so need both as well.
custombuild 1.2 does not have that option, you should install the both versions as PHP-CGI into different locations (change prefix in configure file to install PHP 5.2. and 5.3 in different locations). Then update suphp.conf and update httpd.conf.
Manual modification is needed, sorry have no ready How-To for that.