We don't support to have two liferay installations sharing the same Tomcat and MySQL but we support to install more than one Bitnami Liferay Stack in the same VPS . The main problem is that each one of the Bitnami Liferay Stack will run on different ports. So for example, if you have www.domain1.com and www.domain2.com, you will be able to configure the apache VirtualHost of the first Liferay installation to access using "http://www.domain1.com" but the second Liferay installation will need to be configured to access using "http://www.domain2.com:ANOTHER_PORT" (for instance: http://www.domain2.com:81)
You could try to use a ProxyPass configuration on a third Apache, or include a VirtualHost + ProxyPass configuration on the first Liferay Installation pointing to the second installation. For instance:
configuration for the first liferay installation
ProxyPass / http://localhost:81/
ProxyPassReverse / http://localhost:81/
As you see, it is not an easy configuration, and I would suggest to use different machines for each one of the Liferay Installations.
A part from that, as our Stacks are self-contained softwares you will be able to run any software you want. Our stacks only need to bind some ports, so keep in mind that if you install Liferay as root and you are using the port 80, you won't be able to use it, but a part of that, you will be able to use your system as you would do it without our stacks installed.
About any special requirement for the VPS, the only suggestion that I could give you right now is that you should choose a powerful VPS because Liferay uses a lot of resources... and if you install two Liferay stacks in the same machine, you will need some GB of RAM (4GB should be OK).
I hope it helps.