Michael,
[my scope: SharePoint 2003]
yes, you can deploy multiple XSNs to a forms library, but only one is 'connected' to the "Fill a Form' button of the forms library, this one is named 'template.xsn'.
Use FrontPage or SharePoint Designer to upload the template XSNs to the FORMS folder of the forms library.
With multiple templates for a library also multiple form files (from different templates) are stored in the library.
InfoPath supports field promotion where DataSource fields are promoted to SharePoint columns. When publishing a template, a "properties.xfp" file is created in the Forms folder, which contains xpath query to fields in the forms xml file.
You can edit this file to support field promotion for multiple InfoPath templates, you can even map the same field for different InfoPath template to the same SharePoint column! It takes a little experimenting.
Know that field values turn into column values when a file is saved and NOT when you changed to properties.xfp file.
Good luck.