Buffer Pool and Multiple instances of SQL Server If you have multiple instances of SQL Server running on the same windows server system,

a) Does buffer pool is shared amount multiple instances? 

b) What is the effect of running DBCC DROPCLEANBUFFER OR DBCC FREEPROCCACHE? Would that only affect the SQL Server instance you ran the command on or all the instance(s) on the windows box would flush SQL server cache?
The buffer pool in SQL Server isn't shared across intances.

DBCC DROPCLEANBUFFER removes all "clean" pages from the buffer pool, "clean" pages are pages which haven't been modified since being read into the buffer pool.

DBCC FREEPROCCACHE removes all cached execution plans from the plan cache.