ModelMayhem is a community with two core constituents; models and photographers.
There are many communities where two groups like this come to meet.
For these communities, you need them to self-identify on their first visit. You need to create separate user journeys for each group.
What interests one group won't interest the other. Create different registration pages, confirmation e-mails, and guide them towards participating in discussions most relevant and topical to their unique interest.
Have two distinct e-mail lists too. Send different newsletters to both groups. In larger communities you might have a dozen unique user journeys depending upon the visitor's source, identified interests, sub-interests, or demographic/habit/psychographic variables.
Much more work perhaps, but much more effective.