Currently, users can only select one tag and one category. It would be very helpful for users to be able to add multiple tags and categories.
We have reopened this idea and will consider adding it to our future roadmap.
Until then, you may want to add a custom tag field to your idea form. Portal users can select multiple tags when they submit an idea.
Also, as an Aha! user you can edit an idea within Aha! to assign it to multiple categories.