It is possible to install only the free version, but I'm not sure it is advisable...
Knowing I'm not going to buy Pro for my home machine, I thought I'd avoid the switchover in a month, so I stopped the initial unpacking when it asked me for the four keys. I then went into the plain XT folder (in the location I had chosen when beginning the install) and ran its setup.exe, which worked fine. The program installed and works. From inside DoubleCAD XT you can open any of the file types it knows about. You can double-click 2cd and 2ct files in Windows Explorer.
But... You can't successfully right click a dwg or skp file to open it. I had XT V1 already installed, and it continued to "own" those filetypes even after V2 was installed. (Personally, I expect an "update" to replace the older version, but I guess that was never the intention of the DoubleCAD authors.)
So once I saw V2 working, I decided to uninstall V1 using Windows "Programs and Features". That seemed to work fine, and did not obviously harm V2. But it didn't fix the file associations. I tried a repair install of V2, but it didn't help either.
The bizarre factor was that in every case I'd ever seen, you could always right click on the .dwg file and select Open With, then select Choose Program, and in the dialog box click "Browse". In this case I could browse to the doublecadxt.exe file and select it, but _nothing_ happened! It did not appear in the list of possible programs, but there was no error message either! I tried every route I know and couldn't get DoubleCAD to appear as an "open with" program choice in Windows.
Improvising in the registry, I created
but that didn't help (at least by itself).
and setting it to "DoubleCAD.Drawing.1" seems to have solved the problem for .dwg files. To duplicate the official installer usage, that key needs to be REG_NONE instead of REG_BINARY, but since it seems to work as is, I didn't bother with regedt32 and all the permission and ownership hassles.
Of course that didn't fix .skp and all the other file types... Obviously we shouldn't need to hack the registry. Is it possible nobody has tested the free XT V2 installer without having had the 30-day XT Pro V2 version installed first?
Is there some easier way to get DoubleCAD XT V2 to appear as a right-click "open with" choice?
On the positive side, V2 seems to have fixed some of the maddening dimension entry issues I was having with V1.