[nevcairiel]

The reason to offer a two-in-one installer is that most normal users just have no clue whats best for them, so you take away one source of errors.
It greatly depends on what you are trying to install. A normal GUI application, there is little reason to use 32-bit on a 64-bit system. A plugin for some framework? Probably install both 32 and 64, if that framework commonly supports both.

For my LAV Filters, for example, DirectShow players exist in 32-bit and 64-bit equally. I don't expect a user to know what their player is, so I make one installer that installs both on a 64-bit system, so that its guaranteed to work in whatever player they have.
Of course the installer lets you turn off either, if you are an advanced user and know what you are doing.