Bruce is correct. One in one stream but can be in multiple engagement programs.
Why Can You be in Multiple Engagement programs? Different Content. You may want someone in multiple streams to receive different kinds of communications. For example, a person may be in a 30-day eveal nurture program that delivers specific trial content weekly and another that drips standard nurture content once every month.
Wait, I don't want that. I want someone is one engagement program only If you only want someone in one engagement program, think about setting up a traffic copy campaign that ensures someone is not part of another engagement program before adding them to another.