Letting new users choose role, or just letting users change roles

I’m building a multi user site (not using MU though) and I want to give new users the choice of two custom roles I created with the capabilities manager plugin. As fas as I can tell, I can only choose one default role.
What I’d like is to have two choices on the registration screen, but I can’t figure out how/if this can be done. If that’s impossible, I’d like to allow the user the ability to switch to the other role in the admin section.