The exact best solution for you may depend on the wider context of the nurture and the closer context of the content of the emails, however two solutions I'd suggest are:
- A2 is the second email in the first stream, B1 is the first email in the second stream. You're moved into stream two if you clicked on any/a specific email in stream one, otherwise you stay in stream one.
- Use nested default programs, with all emails in one stream and A1 & B1 both nested in a single default program, the version sent filtered according to past open behaviour.
As an aside, I frequently don't actually use transition rules for nurture programs, and use smart campaigns instead - they're a little more flexible and, I find, work a bit better. If you're keen to run as three separate streams, smart campaigns would be easier to handle transitions.
Regardless, I suspect that #1 will be the best solution for you, it's definitely the simplest if you aren't already using nested programs.
Thanks! I think I was thinking too complicated