Windows will associate files with the latest installed version of that program. However, when using the "Open With" procedure, you should be able to locate and select the appropriate one and not have it "disappear" on you. I've never seen this happen before and suspect it is the result of a recent faulty Windows Update.
 
After I changed the registry, I can now use the "Open With" procedure and select X1-32 AND X2-64 and they both now show up in the list. For some reason though, when I select X1-64 it shows up in the list as X2, so there's definitely something wrong within Windows.