If you'd like to display more than one adbar per page, you should use the #exec cmd version of the SSI call so that each produces a unique adbar. Using the #exec cgi version won't suffice because there is the possibility that both calls will produce the same adbar. There is no way around that using #exec cgi.
However, you can use the shell version to accomplish what you want. To do this you would create two relations, each with a unique set of ads that may be displayed. Each relation should then have a unique rule defined for it, and each AMS call should have one of the rules passed. For example suppose you have two relations setup, one using the rule AD:USE_RULE1 and the other using the rule AD:USE_RULE2, each with different adbars selected as shown in the images below:

AD:USE_RULE1
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AD:USE_RULE2
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Now simply add the following tags to your web pages, and each will display a unique adbar:
<!--#exec cmd="/usr/local/etc/httpd/cgi-bin/ams_shell.pl AD:USE_RULE1"-->
<!--#exec cmd="/usr/local/etc/httpd/cgi-bin/ams_shell.pl AD:USE_RULE2"-->