How do I override theme for registration forms when they have the same form_id?

Excuse me if this is a really dumb question, but I just can't figure it out..

I am using AutoAssignRole 6.x-1.2 and Content Profile 6.x-1.0 to generate 2 custom forms for 2 different classes of members (personal members and organisational members). Everything is great, except that I need to override various things on the two forms and cannot work out how to do so because they seem to have the same form_id eg:

Individual Membership Form ('user_register'):

Organisational Membership Form ('user_register'):

Is there any way to give these forms a unique ID? If not what do I do?