odd OS memory consumption by two nearly-identical SQL 2008 R2 instances
My bet is #2 has Lock Pages in Memory privileges granted to the SQL Server service account. In this case, Task manager show only a minuscule memory usage (SQL Server uses a different Windows API in this mode). All is normal.

(As an aside, if you by "forcing at startup" mean setting min server memory, then that is not what happens. Min means that if SQL server climbs over it, then it won't go under it. It won't force that allocation at startup. )