You could use CB Conditional as Nick suggested to have 2 different profiles, but it's based off field values so you'll need a field that users select what type of profile they want (you can make this read only I suppose and only you can change it if you wanted). You'd then show/hide fields/tabs based off that field (edit fields in Field Management to add conditions). This has nothing to do with login though, they'd still login the same regardless of the field value they selected.