Fitzroy,
as Rishi Shah already has kind of told us, the installer will install both the 32-bit and 64-bit versions of X-Plane, at least version 10.x at least.
For me, the Windows installer still hangs up every time (it has to be an issue of my new Win10 installation...having other updating/installation issues as well), but luckily the Ubuntu version of the installer and the software itself works as a charm.
On my hardware i can find two excecutable files:
1. X-Plane-i386 (this is the 32-bit version of X-Plane 10.x), and
2. X-Plane-x86_64 (this is the 64-bit version of X-Plane 10.x).
I have ever started the programme double clicking the latter alternative and it has worked like a clockwork (for now...).
As to answer to your original question, the X-Plane 10 -installer comes with both the 32-bit and 64-bit versions of the programme. In Linux though, you have to choose between the 32-bit and the 64-bit installers, in Windows you don't have to make that decision.
Enjoy Your Time in the Sky :)
-pkortetj