How can I keep two versions of the same package in archlinux?

I assume you wish to run a specific executable with the old library. Let's call the executable myprogram.

If you place libicuuc.so.55 in a different directory, for instance as /opt/oldlibs/libicuuc.so.55 it is possible to instruct myprogram to use the old library with a command like this:

LD_LIBRARY_PATH=/opt/oldlibs myprogram

The library files can be extracted from the package file (that you can probably find in /var/cache/pacman/pkg).

If this does not solve the issue for how you intend to use the application, you can consider running it in a restricted environment (using chroot) or in a container instead.