Can a user be assigned to multiple channels on pCon.login?

No. Every user can be assigned to only one channel. Otherwise, there might be conflicts regarding the catalog data used – for example conflicting prices for the same product. If you want a distinct group of users to access content from different channels, please create a new channel and assign this channel all the catalogs and settings required for the group under question. Users that are not assigned to a channel cannot use manufacturer catalogs.