To have a single instance of apache serve two domain names (with separate web pages) use virtual hosts:

<VirtualHost *>
...
ServerName www.thayersfarm.com
ServerAlias thayersfarm.com *.thayersfarm.com 
DocumentRoot /var/www/thayersfarm/
...
</VirtualHost>

<VirtualHost *>
...
ServerName www.carmarthenshirehistorian.org
ServerAlias carmarthenshirehistorian.org *.carmarthenshirehistorian.org
DocumentRoot /var/www/historian/
...
</VirtualHost>

to top