In this scenario, I am guessing that it will not block, but read section 4.1.10. Concurrency model in ref doc for more info.
why don't you write a quick POC to test this? a simple facelet with two buttons, each invoking an action handler method in Bean1 and Bean2...
I was at home when I wrote the question, but now I am at my workstation again. I wrote a quick POC for it as you wrote and it work as i hoped.
My Bean2 could perform multiple ajax request during the time it took for Bean1 to complete its request. It dit not have to wait.