I did a side-by-side 32-bit install to 1.8.4 on Win10-64, so got the dialog for config file version conflict and to use the --pcp option for having 2 installs, which I did using the Windows-R Run dialog. After doing this Laz2 does not remember this was set on the previous run and to use the new config directory when it starts. I had to manually edit the startup shortcut to use the option on each invoke.
Also, it would be nice if the installer asked which top menu name/shortcut name prefix should be used when a previous installation is detected but not being upgraded and overwritten, so the shortcuts and menus from existing version aren't affected.
The installer really only supports shortcuts/menu for one install, since (afaik) all shortcuts have the same name. (Of course once you renamend them, it should be fine.
You should NOT need to edit the properties of the shortcut(s) to add pcp.
If you click on "secondary install" you will be ask for a config dir. This config dir is stored inside the lazarus dir, in a file called lazarus.cfg.
Secondary install applies to same version numbers, i.e. 2.0 release and 2.0 fixes; installer was complaining "base version not found" or some such when I tried that option. This "lazarus.cfg" file is not mentioned as what is checked for in addition to the default dir, or its format if someone wants create or edit it manually, that I've seen in the docs. The --pcp option doesn't create it either, apparently only the installer does. It also does not have a location entry in Options/Environment/Files dialog pane, so side-by-sides of the same version can share a config file if they want, for this .cfg or the config file itself directly.
Yes, the menu shortcuts can have the same name, if the menu directory they're part of is named differently. I think (haven't looked at them in detail recently) menu directory name is static strings in .ist files, not variables like install dir. Desktop shortcuts could be handled similarly, afaik.
Given for fresh installs these gotchas don't apply these comments are nits, not bug reports, but would enhance the OOBE for other users of the current version, I imagine, that like me want to test things in the new one for unknown regressions with their own code.