The solution is to extract all the files from one of the 32 bit iso files onto your hd. You then export all the images from the other 32 and 64 bit wims into that. The boot.wim, setup.exe and setup files will all be 32 bit. It won't work the other way round.If you want a choice which to install you will need to put an ei.cfg into the "sources" folder.
But if you do that, then it will only work for custom install, not for upgrade/repair install. If you want to do an upgrade install you will only be able to do that for 32 bit, and you will have to change the ei.cfg to list the edition, e.g.
You can do it with imagex, or GImageX - AutoItScript if you want a gui method.