Yes, you can have two different apache servers on one server, but they can't listen to the same port at the same time. Normally Apache listens to port 80 which is the default HTTP port. The second Apache version should listen to another port with the Listen option in httpd.conf, for example to port 81.
For testing a new Apache version before moving your sites from one version to another, this might be a good option. You just type www.example.com:81 in the browser window and you will be connected to the second Apache instance. I have done this a few weeks ago when I moved from Apache 2.0 to Apache 2.2.
If you want a semi-permanent installation of two Apache versions, consider using the first one as a reverse proxy for the second one, or put both Apache versions on a non-standard port (for example 81 and 82) and use proxy software like Squid to reroute HTTP requests to these two versions.
Yes, you can have two different apache servers on one server, but they can't listen to the same port at the same time.
True, but not entirely. I have run both Apache 2.0.x and Apache 1.3 on the same server listening on the same port, but different IP addresses. Technically I guess one could argue that might be recognized as a separate port ;)