Please do not mark as duplicate because somewhere else someone says that installing
unison-all will achieve the same.
I am not asking "How can I use several versions of Unison in the same machine?". The answer to that would be: install the package
unison-all (which, by the way, is a poor solution because it does not allow to use all versions, but only a few recent versions).
I am asking how to install two specific versions of Unison and use them separately.
Please understand the difference between
How can I use two different versions of Unison in the same machine?
How can I install two different specific versions (from two different specific packages) in such a way that both can be used independently on the same machine?
Note that in the later, you would only be able to use those two specific versions and nothing else, whereas in the fully different first case you would have access to several versions.