When are shared read locks released?

I don't believe that it's acquiring two page level locks at the same time. I think it only appears in profiler that way because the events happen so quickly. if it occurs like you suspect, there would always be two page level locks, but when running a large query with shared lock, I sometimes see two page level locks and sometimes one through this query:

SELECT *
FROM sys.dm_tran_locks
WHERE request_session_id = <SPID>

So, what I think is happening is:

1. acquire: db shared lock, table shared lock, page shared lock
2. page is read... simultaneous release lock on page AND acquire lock on next page

The result of two is that sometimes in the sys.dm_tran_lock query. I'm seeing two PAGE locks and sometimes one and a few times three.. depends on what occurs faster during simultaneous actions.

It's pretty interesting to watch actually, you may want to fire up profiler and trace the lock acquisition/release of some simple queries. I did this awhile back, it was something like: acquire page 1 acquire row 1 acquire row 2 release row 1 acquire row 3 release row 2 acquire page 2 release page 1 ...

I may not be 100% correct, but that was basically the approach. So the lock is released after the row is read, or maybe more correctly it is after the next rows lock is acquired. I suspect this may have to do with keeping a consistent state for traversal.