SQL AlwaysOn Availability 2 synchronous nodes on either side of an asynchronous link
No that is not possible.  The commit mode (sync/async) is between primary and secondary logical roles, and not between secondary and secondary (which is why are you aiming for).

I'm not sure what you'd even gain from that.  All your logical design would be adding (even if it was possible) would be a second hop of log blocks for SQL-B2.  You would want your data to go from SQL-A1 (primary) -async-> SQL-B1 (secondary) -sync-> SQL-B2 (secondary).  Like I said before, that isn't possible.  But what is possible would be SQL-A1 (primary) -async-> SQL-B2 (secondary).

The only thing your projected design would optimize would be network traffic between the data centers.  To reiterate, this discussion is just theory only.  Functionality dictates moving forward.