The best, most complete method for installing multiple versions of the same application involves the use of jails. Keeps things perfectly separated. But it can be overkill for most desktop situations.
You can play around with the "PREFIX" setting to install ports into alternate locations (not /usr/local). Depending on the dependencies, and how many libs the two ports need, this may not always work.
Another alternative is to install one version via the ports tree, and the other version manually, setting the --prefix to someplace totally separate from /usr/local.
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