At user registration/activation create multiple sites with specific themes

Once the user registers and activates I want to create 2 sites( site a and site b) automatically based on two different themes. I want the user to be returned to site a to customize (I will take care of this).

How would I do this?

I'm using BuddyPress, Multisite theme manager, ultimate branding. I'm pretty sure I will use WooCommerce vs ProSites but the final decision has yet to be made on that.