Is it possible to have two Sharepoint installations on one server?
By two installations I mean one is Sharepoint 2010 for Internet Sites and other is Sharepoint Foundation 2010
Any way to do it without having virtual server partitions etc?
In this case I think you will be better served at looking at a utilizing the hosting model for SharePoint. This will allow you to create 'Feature Packs' so you can have one app or site subscription configured with Enterprise or Standard features, and the other one configured for Foundation.
There are other considerations here though. If, you are considering say an Internet / Intranet scenario, and will have sensitive information stored on the intranet side you should consider two farms. These farms should be physically or logically (VM) separated so there is no chance of content bleeding over. Additionally you may want to consider protecting your intranet farm via VPN or Reverse Proxy. While Unified Access Gateway (UAG) is the MS recommended solution, there are plenty of opensource alternatives (if budget is a concern).
Whenever I hear a discussion regarding the "internet" license I like to point out that you should be working with a license specialist, either a VAR or MS directly as you need to plan for more than just that one license.
SharePoint for hosters whitepaper
SharePoint Hosting Architecture Poster
Spence Harbars Rational Guide to Multi Tenancy